Its, without an apostrophe, is the possessive of the pronoun it. It’s, with an apostrophe, is a contraction of it is or it has. If you’re not sure which spelling to use, try replacing it with it is or it has. If neither of those phrases works in its place, then its is the word you’re looking for.
Most English speakers are comfortable with the difference between its and it’s, yet even the most careful writers mix them up in careless moments. Such errors are typos, not grammar mistakes (there is a difference), and can usually be stamped out with a quick proofread. None of us is immune to these mistakes, so let’s not be too hard on people who make the occasional its/it’s slip-up.
In its place is a general sense that the United States and its allies have limited leverage. [Slate]
Columbia University has often shown that the principle is among its core values. [New York Daily News]
The rest of its funding comes from state and federal grants and private fundraising. [AP]
It’s a powerfully expressive grape that asserts both strong fruit and structural characteristics. [Huffington Post]
Apple left little doubt that it’s poised to reveal the iPad 2 at an event in San Francisco to be held on Wednesday. [USA Today]
The courses began in June, and since then, it’s been booming. [Charleston City Paper]
Note that in each of these three examples, it’s would bear replacement with either it is or it has. In the above three examples, neither of these phrases would work in place of its.