Their, There and They're

They're over there, they've bought their picnic!

They're is self explanatory: It is a short form of "They Are"

Their means belonging to them. A good way to remember the meaning of this word is that it has a little person in the middle:

so it is a good way to remember that it is their item.

There is the opposite of Here. It is a place, Here or There. 

Once you have learned the differences of these three different words, then your brain will recognise them by the meaning and it will jump when you see it spelled wrong in the wrong place.

Their over They're, they have bought there picnic. Doh!