Possession

"Possession is nine-tenths of the law!" is an English idiom. This idiom expresses ownership. It means that if you have something in your possession (in your house, or business, or somewhere you can get it) you have a stronger legal claim to it than someone who doesn't have it.

An example: Josie and Freddie fight over a sofa

Josie and Freddie were roommates. Josie bought a sofa for cash for their living room. Josie moved out but left the sofa behind. Freddie told Josie that she could get it later, after she got settled.

Three months later, Josie went back to her old apartment to get the sofa.   
    
But Freddie didn't let her take it. He said, "The sofa belongs to me. The sofa's owner is me. It belongs to me. The sofa is mine. The sofa's mine! It is my sofa. It's mine.  You can't have it. Possession is nine-tenths of the law, and you can't prove that it's yours."
    
Poor Josie! Freddie is right. Josie has no proof that she bought the sofa - no receipt, no people who saw her buy it. It will be difficult for her to get the sofa back.
    
When you are learning English, knowing how to speak and write about possession is important. Freddie knows many ways to say the the sofa belongs to him (even if it doesn't!) If Freddie were a nice person, he would say, "The sofa belongs to you, so you can take it. The sofa's owner is you. The sofa is yours. It is your sofa. It's yours. The sofa's yours!" 

Possessive Pronouns and Adjectives

The words and phrases that show possession in the statements Freddie makes are belongs to me, mine, my. If he were a nice person, he would use the words and phrases belongs to you, yours, your. 

Their are three types of words that show possessions. They are:

Possessive pronouns (mine, yours, his / hers / its, ours, theirs)
Possessive adjectives (my, your, his / her / its, our, their)
Possessive nouns (add an apostrophe plus s to the noun - example: the sofa's owner, Nina's book, my mother's name.)

In addition, we can show possession by using object pronoun phrases
(belongs to me / you /him, her, it / us / them)

Links

Here are links to pages about Possessive pronouns, adjectives, and nouns.

A simple description on form and use (studyzone.com)
Possessive Adjectives and Possessive Pronouns
A nice comparison that shows why they are confusing. (eslcafe.com)
Possessive's
A nice description of possessive nouns (englishclub.com)
Possessive Nouns
Simple explanation. (funeasyenglish.com)