Si usano per indicare l'appartenenza di un oggetto.

E.g.:

La mia macchina. --> My car.
La nostra macchina --> Your car.
etc.
Si usano quando ci si riferisce a se stessi (me, us) o ad un altra persona in generale (It's you, him, her, it, them)

E.g.:

Who is it? It's me./ It's us.
Mi/Ci ha spaventato --> It gave me/us a fright.
Who's at the door? It's them.

Mine, yours etc. si usano per indicare un oggetto che appartiene a qualcuno o quando si domanda a chi appartiene qualcosa.

E.g.: Some friends of mine/yours/his/her/theirs.

Whose is it? It's theirs.

Esempi finali:

Which one is your car? Mine is the one right there, my father gave it to me for my birthday.
Where are my keys? I don't know, it's you who lost them, mine are right here!