It's and Its

It's means it is or it has.

Its means belongs to it.

Whenever you write a sentence containing it's, pause and read it saying the it's as "it is".
If it doesn't make sense, read it as "it has". If it still doesn't make sense change what you've written to its.

It's and its are often confused. The apostrophe in it's indicates the omission of the letter i, or the letters ha. It's can be short for it is or short for it has.

The word its is quite different. It is related to words such as hers, ours, yours and theirs, none of which has an apostrophe.

The spelling confusion bewteen it's and its partly arises because in English 's (apostrophe s) can donote the possessive: dog's dinner, cat's whiskers and so on. So it's easy to see how the confusion may arise.

Writing it's when its is meant is bad enough, but is perhaps a more understandable sin than putting an apostrophe before the s in a plural.

Correct Use of Apostrophe