I only ask because every day on Twitter, Facebook and elsewhere I see dozens of people getting the two mixed up.
It doesn't matter so much when chatting online to friends but if you're tweeting from a business account it makes you look unprofessional. Or if you're commenting on a blog or news story it can distract the reader from what you're trying to say. So here's a quick guide.
You're is short for you are.
For example: 'You're really great at making cakes.'
Not: 'Your really great at making cakes.'
Use your when talking about something which belongs to somebody else.
For example: 'Mmmm, your cakes are really delightful'.
Not: 'Mmmm, you're cakes are really delightful.'
If in doubt, say the sentence out loud with the words 'you are' instead. So, 'Mmmmm, you are cakes are really delightful' just sounds wrong doesn't it?
Another thing to remember is that 'your' is never followed by 'a'. Unless you want to say 'Your A level results are better than mine.' Or 'Your A-bomb is making me nervous.'
Here's some common mix-ups to avoid:
'It's great to see your back.'
(Yikes, this shirt is see-through. )
'Just got you're email'.
(You just understood that I am email?)
(This is just an open invitation to pedants to cry, 'My welcome is what?')