IT'S = IT IS, always. When you use IT'S - with an apostrophe, it means IT IS. ALWAYS, NO MATTER WHAT.
Examples (the first two are incorrect; the last three are correct):
The dog lost it's (it is) bone.
The site is notable for it's (it is) collection.
It's (it is) a story of two cities.
We think it's (it is) easy.
It's (it is) only a dream.
I do understand why people do this: that fuzzy rule about possession, i.e., use an apostrophe to show ownership.
Its = possessive pronoun, that means it was created specifically so that you wouldn't have to use an apostrophe to show ownership.
Examples:
The dog lost its bone.
The site is notable for its extensive collection of links to resources.
It's not so difficult, is it?
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