1. Use "s"or "es" to show plurality in countable nouns. You need to show plurality when you are talking about more than one or are speaking in general terms about all of the items in one category. 

2. Use "s" for present tense subject/verb agreement. Add "s" on the end of a verb in present tense to agree with the singular "he," "she," or "it" subject .

3. Use an apostrophe followed by "s" ('s) to show that a singular noun belongs to someone or something.

4. Use an "S" followed by an apostrophe (s') to show possession of plural nouns or nouns that always end in "s." 

The proper noun, Les, always ends in "s." Notice the apostrophe is added after the s.  For proper nouns ending in "s," it is also accepted to add 's (Les's book).

5. The word "is" is often abreviated (or "contracted") in English. Use an apostrophe followed by an "s" ('s) in order to show the contraction for the word "is." 

"IT'S" is simply a shorter way of saying IT IS. There are several other words that are commonly used with 's to show a contraction. For example, who's, what's, where's, there's, he's, she's, etc.