Words that show ownership or possession.
Strictly, a possessive pronoun stands in for a noun (Das Buch ist meins), while a possessive adjective stands before one (mein Buch). Learners usually meet both together, and they share the same stems.
The stems
Singular: mein (my), dein (your), sein (his), ihr (her), sein (its).
Plural: unser (our), euer (your, plural), ihr (their). The formal "your" is Ihr.
The stem depends on who owns the thing. The ending agrees in gender, number, and case with the noun that is owned, not with the owner.
Examples
Before a noun: Das ist mein Buch.
That is my book.
On its own: Das Buch ist meins.
The book is mine.