Pronouns replace nouns. They can be singular or plural. Pronouns for people are I, we, they, he, and she. The pronoun for objects is it. Using pronouns in a sentence are easy because they simply substitute the nouns. For example, in the sentence, Mike is a beautiful boy. You can replace Mike with He. The meaning of the sentence remains the same. More examples in using the pronouns for sentences follow.

I am gorgeous. We are going to the mall. They are the baby’s parents. He is always present. She loves to read books.

In using nouns for these sentences, they can appear as follows.

Mags is gorgeous. Mags and Shella are going to the mall. Ben and Ingrid are the baby’s parents. Ben is always present. Jessica loves to read books.

A pronoun is a word that replaces a noun in a sentence or a subsequent sentence. If we used the main noun by its proper name in writing (or in speaking), that would be horribly redundant and awfully hard to understand. For example, “Susan got Susan’s car back from the mechanic because the mechanic told Susan that the mechanic would have Susan’s car done in time for Susan’s date on Friday.” See how silly that sounds? Isn’t this much, much better? “Susan got her car back from the mechanic because he told her she could have it back in time for her date on Friday.” “She” and “her” refer to Susan, and “he” refers to the mechanic. Without pronouns, we would be stuck with writing the first sentence. And tripping over ourselves all the time!
Here is a chart showing you most of the pronouns:
http://grammar.ccc.commnet.edu/grammar/cases.htm

A pronoun is a word that replaces a noun. Most pronouns are used in one of two ways. The first way pronouns are used is to replace nouns. The second way pronouns are used is to refer to specify nouns. Pronouns are most often used in situations where using a noun would be cumbersome or awkward.

There is also a group of pronouns known as personal pronouns. They replace proper names in sentences spoken in first person.

Here are some examples:

Mary is my best friend. She likes soccer.

The pronoun “my” replaces my name in the sentence and needs to as this sentence is spoken in the first person. “She” in the second sentence refers back to (and replaces) Mary in that sentence.

I love chocolates. These are my favorites.

In the second sentence, “these” refers to which chocolates are my favorites. It replaces the proper term for the chocolates that are my favorites.

From Wikipedia.com

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In linguistics and grammar, a pronoun (Lat: pronomen) is a pro-form that substitutes for a noun (or noun phrase), such as, in English, the words it (substituting for the name of a certain object) and he (substituting for the name of a person).
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You can read the whole article here: http://en.wikipedia.org/wiki/Pronoun