Traditionally used as a plural possessive pronoun, "their" has come to be a shorthand for saying "his or her."

Before the women's rights movements in the U.S. it was considered acceptable to say "his" when the gender of a person was unknown but women took offense to this and demanded that people say "his or her" instead. This idea was adopted for a while, and it was common to see people writing shorthands such as "his/her," but people eventually grew tired of this and started saying "their" instead. It was not long before English professors and teachers became upset by the new trend, and to this day you can still hear them scream when you use it in a paper, but that's really okay because it's their fault for making us say "his or her" instead of "his."

"Their" represents possessive.

Therefore,

Favourite way for internet retards to spell the contraction "they're" or they are.

Property of multiple people.