It's and it's are two very different words that are commonly misused. 

It's is a contraction. It's is short for the two words, it is. Let's look at an example.

"Oh! It's SS501's Kim Hyun Joong!"

Here, it is saying that the person is Kim Hyun Joong. It is Kim Hyun Joong. If it wasn't used as a contraction, it would be grammatically incorrect and it would somehow explain ownership of him, which leads us into the word its. 

Its is a word used to show ownership of something. It is used to pronounce ownership of traits, items, feelings, etc. Let's look at another example.

"Jonghyun's dog, Roo, must be happy! Its tail is wagging."

This is a perfect example of ownership. It shows that it is Roo's tail, and more importantly, its is used in a way that the person talking is showing that they do not know the exact gender of the dog. Its is used often when the person speaking does not know the gender of something. I personally don't know Roo's gender (bad Shawol! ><) so I used its in place of his or hers. 

Just remember that it's is a shortened word for the two words, it is, and that its shows ownership.

Love always,

love4kpop827 aka, Cinderella