A pronoun is a word used as a place-holder for a noun, a noun phrase, or a different pronoun. Pronouns are usually used in writing and speech as a way of keeping the flow of the words smooth by reducing repeated use of the full subject or object word.
Pronouns usually come after the noun they are replacing, as in the sentence:
Lily smiled down at him, and as she did her hair fell in front of her face.
In some regional speech and less formal writing, the pronoun may come before the noun it is replacing, but in this case it should be made very explicit which noun is being substituted. A dialectical example of a precedent pronoun is:
She smiled down at him, Lily did, while her hair fell in front of her face.
It's important to note that the instances of her in the examples above are not pronouns, specifically possessive pronouns. Rather, they are possessive adjectives.
There are many different types of pronoun, but the most important include the personal, the demonstrative, the relative, the interrogative, the indefinite, the intensive, and the reflexive.
A personal pronoun replaces a specific thing or person, which may be the subject or object of the sentence. A personal pronoun may also indicate possession. The usage of the personal subject pronoun and the personal object pronoun is one of the lessons many people must learn when trying to speak grammatically, as typified in the I/me situation. The word I is used in situations in which the first-person personal pronoun is replacing the subject of the sentence, while the word me is used when the first-person personal pronoun replaces the object of the sentence.
The common confusion comes when another person is paired with the first person pronoun, and speakers are uncertain whether to use I or me. For example: the sentence John and me went to the garden. is ungrammatical, because the pronoun in this case is replacing the subject of the sentence -- John and I went to the garden, the garden did not go to us. On the other hand, in a sentence such as The waiter gave our food to John and me. the use of me is proper -- in this case the waiter is the subject of the sentence, and both food and the phrase John and me are objects.
For most pronouns, such as we/us, people have no trouble determining when to use the objective case and when to use the subjective case -- it would be strange indeed to hear someone say Us went to the garden. The only other case that gives most people difficulty is who/whom, which has been mostly resolved by the acceptance of who for both the subjective and objective cases.
In addition to the personal pronouns, a pronoun may be used to replace many other types of nouns. Demonstrative pronouns, for example, serve to refer to nouns which are either near or far in space or time. The demonstrative pronouns are this and these for nearby nouns, and that and those for faraway nouns.
Relative pronouns are the words who, whom, which, and that. They are used to join together two different clauses or phrases, as in the sentence The writer who has the most talent is not necessarily the one published.
Interrogative pronouns are used to ask questions. These pronouns are which, what, who, and whom. An example can be seen in the sentence Who is your favorite actor?
Indefinite pronouns are often actually indefinite adjectives, and the classification depends greatly on who you ask. The list of indefinite pronouns is quite long, but in essence they are pronouns that refer to a non-specific thing. An example would be in the sentence John gave Glenda everything from the car.
Intensive pronouns are used to reinforce the noun or pronoun they come after. Intensive pronouns are not commonly used in modern writing or speech, except for formal occasions and speeches. An example of an intensive pronoun is in be the sentence He himself did what was needed.
Reflexive pronouns are used to explicitly refer back to the subject of the clause. These are words such as himself, herself, yourself, themselves, or myself, as in the sentence John remembered her fondly, later asking himself where it all went wrong.
Pronouns are an important part of English, helping to keep sentences flowing without being bogged down in verbiage. It is important to remember, however, that a pronoun is only as useful as it is easy to track back to its source noun. Sentences with multiple objects and repeated use of pronouns that could refer to any one of the objects should be avoided -- pronouns are meant to improve comprehension, not to detract from it.