Words like the following are called pronouns.


ich, er, diese, man, welches, einer, meine, jemand, keiner, sich



Pronouns substitute a noun or a noun phrase in order to avoid unnecessary repetitions.


Meine Schwester hat meinem besten Freund Thomas das alte Fahrrad geliehen.  
Er bringt es ihr morgen zurück.

use of a pronoun

Mein bester Freund Thomas bringt meiner Schwester das alte Fahrrad morgen zurück. repetition of the noun phrase



Certain pronouns, like, for example, the possessive pronouns, can also be used as articles.


Dieses Auto ist meins. used as a pronoun
Dieses Auto ist mein Auto. used as an article



Pronouns are classified, according to their function and meaning, in various categories.


pronoun categories
personal pronouns
reflexive pronouns
reciprocal pronouns
possessive pronouns
demonstrative pronouns
interrogative pronouns
relative pronouns
indefinite pronouns
pronominal adverbs



Pronouns are inflected. There are, however, certain pronouns like man, etwas, nichts, selbst that are invariable.