The apostrophe is used to show that a letter is 'missing'. This is most common in contractions:
I'm, he's, we'll, don't, can't, won't and o'clock (of the clock) etc.
The apostrophe is used to show possessions. Here are the ways this is used:
Add 's to a singular noun:
The mobile that belongs to Mary - Mary's mobile.
The car that belongs to my neighbour - My neighbour's car.