How are definite and indefinite articles used in English grammar?
Indefinite articles are used to talk about any object within a group, definite articles are used to talk about a specific object.
English's indefinite article is 'a' (or 'an'), and definite article is 'the'.
1. Can you pass me a banana?
2. Can you pass me the banana?
In 1, you don't care which banana you get as long as you get a banana. In 2, you want a specific banana, maybe the one you're pointing at, or that came up earlier in the conversation.
There are other uses of indefinite and definite articles which you can read about more here: http://www.englishpage.com/articles/a-an-vs-the.htm