There are two types of personal pronouns: subject and object.
Subject Pronouns - Subject pronouns (I, You, He, She, It, They, We) replace the name of the subject in the sentence. Example: Mrs. Yen did not come to school yesterday. She had to go to the doctor. "Mrs. Yen" is the subject and "she" is the subject pronoun.
Object Pronouns - Object pronouns (Me, You, Him, Her, It, Us, Them) take the place of the object in the sentence (the noun that receives the action in a sentence). Example: After Colleen bought a bike, it got stolen within a week, "it" is the object pronoun used to replace "bike."