I've actually put some time into thinking about this and I think the most basic use of "the" and "a/an" has to do with what the speaker/writer assumes about what the listener/reader already knows.
A common explanation is that "the" is for definite references, but "a/an" is used for indefinite. Though this may sometimes be the case, it isn't always.
Ex. "I bought a car today." is most certainly not just any car. It's the exact car that the speaker bought. The speaker is saying, "I bought a specific car, but I assume you don't know which one I'm talking about."
Ex. "I bought the car today." which is saying, "I bought a car and I assume you already know which one I'm taking about." (Probably because the speaker has mentioned it to the listener before.)
So if the speaker is wrong in their assumption that the listener already knows, the listener would normally respond, "What car?" In other words, "Please tell me which car you are referring to, because I don't know."
[The text below is in response to Araucaria's comment. -- Thanks, btw.]
The zero article is the plural for "a/an", but I think it points to a category.
Ex. "I like fast cars." = "I like things that fit into the category of fast car."
Ex. "I like cake." = "I like food that fits into the category of cake."
The plural for "a/an" = "speaker assumes listener/reader doesn't know" is "some".
Ex. "I bought some pretzels on the way home" (assumes listener doesn't know which "pretzels")
The plural for "the" in this case is "these/those".
Ex. "I bought those pretzels on the way home" (assumes listener does know which pretzels are being referred to)
Note also that if the speaker's assumption is wrong, the listener will usually respond, "Which pretzels?"
There are some other functions of "the", "a/a", etc. as well, but I can't remember what they are at the moment.