Demonstrative pronouns translate to the one(s), or that/those when replacing a noun. There are four forms, but they are not often used alone. De, qui, que, dont and -ci or -là usually follow them.
Donnez-moi mon billet et celui de Guillaume. Give me my ticket and William's. (or: the one of William)
Il porte ses propres livres et ceux de sa sœur. He is carrying his own books and his sister's. (or: those of his sister)
Quelles fleurs aimes-tu, celles-ci ou celles-là? Which flowers do you like, these (ones) or those (ones)?
Ceux qui travaillent dur réussissent. Those who work hard succeed.
C'est celui dont je parle. That's the one I'm talking about.
The indefinite demonstrative pronouns ceci (this), cela (that) and ça (this/that) refer to indefinite things or ideas.
J'aime ça. I like that.
Prenez ceci. Take this.