I’ve seen this error so many times on the net – in articles, in blogs, in the front page of so many web sites – that I think I’ll spare a few words for it. The subject is of course, English Grammar, and for ppl wh0 typ3 l1ke th15, this is not a consideration. Real people may read the next paragraph.
I, T, and S can form two combinations in that order. Its and It’s: i.e., a simple I, T and S, and I, T, an apostrophe and an S. The difference between them is that ‘Its’ is a posessive pronoun, where as ‘It’s’ mean It is.
Correct: It’s a nice day. Why? Because it expands to ‘It is a nice day.’
Wrong: It’s name is Dolores. Why? Because it expands to ‘It is name is Dolores.’
Correct: The cat had its whiskers cut. Why? Because ‘its’ is a posessive pronoun here.
See also: It’s/Its