There is an adjective.
Meanings: in a place, could be nearby or at a distance. It could also mean present or in attendance.
Their means belonging to them.
They're is a short form of they are.
Examples of There:
1. There is a tendency amongst children today to get addicted to Internet games.
2. The coach placed a green cone at the other end of the field and told the players to run till there.
Examples of Their:
1. Manish and Shravan called all their friends home to celebrate their success in the CAT exam.
2. The doctors were told to come with their families for the Hospital's annual day function.
Examples of They're:
1. "They're not there" Nikhil informed his uncle when he was asked if his cousins were on the terrace.
2. Two members of the group informed the leader that they're not involved with the project.