I have seen and heard both, but I cannot tell the difference between "meaning of life" and "purpose of life".
Could anybody explain it to me? When to use which?
There are both similarities and differences between the two expressions, but each has several things in common: abstractness, universality, human nature, and human nurture.
People are creatures of language. We create, use and misuse words, or symbols, to comprehend our world as mediated by our senses. At this point, we need not concern ourselves with controversial and widely divergent views on why and how this happens. Let's just make it a presupposition that is at least somewhat tenable.
That people are "languaging" critters who acquire and use words to make sense of the world into which they are born is pretty clear. If words were to change their nature and content from day to day, life would be unnecessarily difficult. If today the word cat is a furry animal with four legs that says "meow" but tomorrow is a slimy green animal that says "ribbit," life would be very confusing indeed. That's why people universally, regardless of their heart language, agree that whatever word is assigned to that furry, four-legged critter that goes "meow" will always be cat (or gatto, chat, katze, kissa, ad infinitum).
Here's where the word meaning enters the picture. At their most basic, words correspond to some tangible reality. For example, an infant who is hearing a parent reading a picture book to her will hear the word cat, see a picture of a cat, and before you know it, upon seeing a live cat will point to it and say, “Cat!” At this (perhaps) most basic level, meaning begins to emerge in the human species. As our vocabularies and syntactical abilities increase, so does our comprehension of the world around us. We start making linkages between words and groups of words, and before you know it we have constructed a fairly workable, symbolic version of reality. That "reality" has meaning to us.
On the nurture side of things, our parents, by word and by example, teach us to make the transition from the nexus of words and things to the nexus of words and ideas (or concepts). A baby cries when it is hungry and mommy gives her a bottle. A couple years later, the infant feels hunger, and instead of crying says to her mommy, "I hungry," and her mommy feeds her. A few years pass, and the same child resists stealing candy from a store because she knows it's a "bad" thing to do. Notice the quantum leaps, from 1) crying for food to satisfy a simple physiological need; 2)asking for food through words (including “please,” if you’ve learned your manners); and 3)refusing food out of respect for a moral code and out of fear of punishment perhaps.
As we mature and interact with other living and sentient beings, we begin to realize that we all share many common meanings, and this knowledge enables us to use language as a tool for communicating with each other. When we say to a person, "I just love petting my cat," the other person may concur with us and say, "Yeah, I also like to pet my cat when it's sitting on my lap. I find it soothing." You can be pretty sure both people have a shared meaning of what it's like to pet a house cat.
Where things get really interesting, though, is when groups of words are no longer linked just to obvious and tangible realities but to intangible and abstract realities and meanings. These meanings and linkages may be significant, but they are also imperfect and distorted, since any description of virtually any aspect of reality is bound to be part reflection, part selection, and part deflection. In other words, our finite ability to describe reality--reflection; our tendency to pick and choose aspects of reality--selection; and our proclivity either consciously or unconsciously to ignore reality—deflection; all simultaneously both describe and distort reality. After all, the human species is sentient, not omniscient!
Despite these obstacles, our two cat lovers might then continue their dialog as follows: "I hear that having a house cat as a pet can actually help lower your blood pressure," to which her friend says, "Yes, XYZ Medical School did a study last year of some 10,000 people, half of whom didn't have cats, and half of whom did, and do you know what the study determined?" They then go on to discuss the benefits of owning a cat.
As you can tell, there is a vast difference between an infant pointing to a cat and saying "cat" and two grown-ups conversing on the physiological benefits of cat ownership. Meaning exists in both situations in that each involves the concrete and tangible element of cats, but the level of abstraction is vastly different between the two scenarios.
What does all this have to do with the "meaning of life" and the "purpose of life?" Well, our two cat lovers could just as easily (perhaps) continue their discussion by employing ideas and concepts even more abstract. They could talk, for example, about what a good thing it is to have one's blood pressure lowered by simply owning a cat. "What if such a simple thing can extend life and enhance a person's enjoyment of life?" one cat lover might ask. "That's a good point," the other cat lover replies. The assumption they share, of course, is that a long and enjoyable life is preferable to a short and miserable one!
Skip 62 steps in reasoning, and you might come up with a number of conclusions regarding these three scenarios: pointing to a cat, talking about cats, and discussing what it means to have a longer, healthier and happier life with a cat versus a shorter, less healthy, and less happy life without a cat.
First, abstraction exists in all these interactions, but there is a quantum leap of abstraction when we step back in a self-reflexive mode and speculate about the meaning of life. Is it "true," we might ask, that a longer, healthier, and happier life is a good thing? If yes, why? How might this "good thing" be linked to some purpose that transcends merely having a longer, healthier, and happier life? As Peggy Lee asked in a popular song, "Is that all there is?" If so, who am I and why am I here? Do I have a purpose in living? Is it a good thing to give back at least as much as I take, thus leaving the world a better place than when I found it? One of our cat lovers, for example, might volunteer a significant amount of her time doing volunteer work for an animal shelter. A Mother Theresa, on the other hand, devotes here entire life to caring for the poorest of the poor in India.
One of America's most significant founding documents, the Declaration of Independence, answers some of these questions this way: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed . . .."
Meaning and purpose. They are certainly intertwined, but exactly how and why and to what extent is a question for the ages. Some people find answers to both the meaning and purpose of life in their belief in a Supreme Being in whose image we are created and whose creative design of the universe bears witness to an intelligent Master Designer.
Some other people find answers to the meaning of life in a "Cosmos" (Carl Sagan) or "a soup of molecules [that evolved] into the biosphere we see today" (Stuart Kauffman). According to them, this cosmic soup always was and always will be. No Supreme Being is responsible for the cosmos' existence and intricate design. A popular saying today is, "It is what it is" [and that's all that it is!].
Most people try not to think too deeply about these things. Perhaps the vast majority of the peoples of the world are more concerned with basic day-to-day survival than with questions of meaning and purpose. The questions are nevertheless important and beg to be answered.
The the meaning of life arises, at least in part, out of a combination of symbolic abstractions. These abstractions comprise a meaningful whole that is greater than the sum of its parts. According to one's perspective, the whole could be a spiritual, a material, or a spiritual-material gestalt. Meaning, in turn, births a cosmology, which is a theory of reality, based on design.
The concept of the purpose of life also arises from high-level abstractions comprising a meaningful whole. This meaningful whole is an interpretation of teleology, which is a theory of why the universe exists and to what end or purpose it exists, if any.
In conclusion, at one end of a spectrum are people who believe sincerely that there is no "meaning of life"; it is what it is, and nothing more. Since I do not place myself at that end of the spectrum, I find it interesting that even people who insist there is no meaning of life live as though it has meaning, and to some extent and for whatever reasons they participate in the social construction of reality.
At the other end of the spectrum are some people--myself included--who believe there is both meaning and purpose of life. We find meaning and purpose in a verse from the Bible. In the Gospel of John, chapter one and verse one we read: "In the beginning was the Word, and the Word was with God, and the Word was God." The "Word," from the Greek logos, is the root of our -ology words, such as cosmology and teleology! At the heart of the word logos are the notions of both meaning and purpose. The two go hand in hand. How? Again, that's a question for the ages.
As to your question, it is a good--albeit philosophical--one. How we use the English language (or any language) at this very high level of abstraction is important. I hope my long-winded answer helps you in some way.
The "meaning of life" is generally intended to describe the overarching reason. Why are we here? What is it all for? Why were we, as people, put on this earth? Why are we alive? What is the meaning of life?
The phrase, "purpose of life", is essentially synonymous; however, purpose of life is used considerably less often than the "meaning" usage, and there may be a regional difference in usage. According to Ngram, "meaning of life" is used approximately three times as much in recent years as "purpose of life", and personally, in my every day life in the Northeast region of the United States, I cannot recall hearing the latter phrase used in any sort of conversation – "meaning of life" is always the phrase of choice, at least in my area.
The concept of a "life purpose" is more often used in the context of individual meaning in life (in this case, it is usually written as a "purpose in life", or as a "life purpose"). It means more or less the same thing, except instead of dealing with the question, "Why are we here?", it deals with the question, "Why am I here?", or, perhaps more accurately, "What do I want to do in life?", or "What do I want out of life"?