The is simply just stating that *there is something, or you could say 'it's over there'.
Their: indicates possession.. Like something is *theirs.
They' contraction of 'they are'. *They're my mum and dad. I could also say *they are my mum and dad.

I often see people getting it wrong. Some people don't take to it like a duck to water. I am terrible with maths, so I suppose I don't understand numbers like some people can't comprehend this. I'm sure there's an explanation for why they're getting this wrong. Everyone has their strengths and weaknesses.

People often mistake: your for you're. Like I received a text which said 'your one of my best friends', as sweet as it was! People can't help it, but for anyone who does mistake the two, you're is a contraction of you are and your also indicates possession... An example would be *isn't this yours?