Articles

Articles are used to introduce nouns.  There are three articles:

1) indefinite articles ("a" and "an").  We use these when:
a) the noun is not specific or we are mentioning the noun for the first time:

- e.g. "A woman is walking down the street." (I do not know who she is, and I am mentioning her for the first time)
- e.g. "I put an apple on the table." (It could be any apple, and I am mentioning this specific apple for the first time.)

b) the non-specific noun is singular and countable:

- e.g. "A giraffe, an emu, and a horse are in my front yard." (We can easily count each one of these animals, and I am mentioning for the first time that they are in my yard.)

2) definite article ("the"). We use these when:
a) the noun is specific:

- e.g. "The house next to my house is red." (We know specifically the house about which we are talking.)

b) we have previously mentioned the noun:

- e.g. "The giraffe, the emu, and the horse are still in my front yard." (I mentioned the giraffe, the emu, and the horse in example 1b; you have already heard about them.)

3) zero articles (i.e. we do not use "a," "an," or "the" and we make non-count nouns plural). We use these when:

a) we are speaking of a type of noun in general:

- e.g. I like apples. 

b) the noun is non-count and non-specific:

- e.g. Coffee stimulates the nervous system.

Resources

University of Victoria and The University of Texas at Austin have good, simple explanation of articles.  Purdue OWL has a good sheet on how to use articles.