Is there a difference between "think of something" and "think about something"? I've also met "have heard of/about something".
In many contexts, think of and think about are effectively interchangeable...
"They say Greece may leave the Eurozone. What do you think of/about that?"
"I'm thinking of/about looking for a new job"
In other contexts, to think of something means you're at least aware of the thing, but may not have given it a great deal of consideration. If you think about something this normally implies more focussed or extended attention.
"I never thought of doing that!" (that possibility never crossed my mind).
"Have you thought about my birthday present?" (have you considered/decided what to get me?)
Much the same distinction applies to hear of/about. You might say you've heard of something meaning no more than that you're aware "something" exists. But if you've heard about something the implication is you've heard some important/current information about that thing.
"I've heard of Amy Winehouse" (the name is known to me, but I don't necessarily know any more).
"I've heard about Amy Winehouse" (strongly implies knowing of her untimely death).
There is a difference between "think of" and "think about."
Some examples of "think of":
A: "What did you think of Mary Jones's speech?"
B: "I didn't think much of it."
A is asking B's opinion on Mary Jones's speech. B replies that she did not think it was a very good speech.
A: "Have you thought of packing water bottles?"
A is asking if it has occurred to B to pack water bottles.
"Think about" has a different meaning. It implies a longer period of contemplation on the matter at hand. Using the same examples, the phrase changes the meanings:
A: Did you think about Mary Jones's speech?"
B: No, I haven't thought much about it yet.
In this case, it is likely that A and B have previously discussed forming some kind of opinion on Mary Jones's speech. A is asking B if she has come to an opinion yet. B is replying that she has not had time to form an opinion yet.
A: "Have you thought about packing water bottles?"
This question suggests that A and B had previously discussed the possibility of packing water bottles, and A is asking B if she has given the matter any more thought.
"Think about" is used when you're actively considering something or concentrating on the thought of it.
Ex. Let me think ABOUT it.
On the other hand, "think of" is used when the idea comes to you.
Ex. I never thought OF that.
In most other cases, they are very similar and can be used interchangeably. Especially for actions in the future.
Ex. I'm thinking ABOUT/ OF going on a trip.
"Hear of" is used when you are talking about the existence of something.
Ex. Have you heard OF that company?
On the other hand, "hear about" requires that you already know the existence of something. You're just being told further news.
Ex. Have you heard ABOUT what happened in Ralph's company?