Pronouns

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.

Examples
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.

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.