This should be a popular one as it’s a very common set of mix-ups (and I know it’s not a pair, but a trio: think of it as a Bank Holiday Bonus)
There – regarding a place. “Have you been to Wales? Yes, I’ve been there.”
Their – possessive of “they” – belonging to them “Don’t take their wellies without asking!”
They’re – less formal, abbreviated form of “they are” “Are Linda and Freddie going to be at the party? Yes, they’re going to the party!”
So, if some people are visiting a female relation who lives on the other side of the valley: “They’re visiting their aunt, who lives over there”.
That’s it, really!
For why we don’t add an apostrophe to “theirs”, see my article on the apostrophe.
For more troublesome pairs, have a look at the category cloud to your lower right, or click here!
LibroEditing proofreading, editing, transcription, localisation 
Arthur
June 3, 2011 at 10:41 am
It’s funny, this. It’s one of the things my English girlfriend does wrong quite a lot (esp. in SMS) and then blames it on ‘typing fast’. Rubbish, I say – you can also type the correct word fast, if you thought about it a sec. Even I don’t make those mistakes, and English isn’t even my first language! 🙂
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Liz at Libro
June 3, 2011 at 10:44 am
Thanks for your comment, Arthur. It is an easy one to miss, and I see it lots, even among people I’d expect to know the rule …
Actually, thinking about it, I seem to only see it with English native speakers; I work a lot with ESOL speakers and that’s not one they tend to make. They probably don’t seem so similar and thus interchangeable if you’ve had to learn English as a second language?
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lindyb
November 17, 2014 at 9:23 am
Please could you add something about using “theirs” as a possessive pronoun? The i Paper, which last week used “their” for “Fred and Tom were going their” (aaaaargh!) today had “as much for your benefit as for their’s” (aaaargh again!)
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Liz Dexter
November 17, 2014 at 9:37 am
Oh dear! I have covered this in my post on apostrophes, but I’ll add a link to that in this post, too!
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