Where are apostrophes mostly used?Explain briefly.

the apostrophe was used in place of a vowel letter .... For most singular nouns the ending 's is added; e.g., the cat's whiskers

There are two main case to use apostrophes as follow:

1. Apostrophe is used for showing possession which means to own something or belongs to.

- The bird's cage was cleaned by my brother. (It comes after singular noun and added s)

- It is Andy's book. (It comes after pesonal name and added s)

- She runs women's clothing business. (It comes after plural noun and added s)

- Charles' Hospital is the best hospital in this city. (It comes after name of place which end in s)

- She needs a new  shoes' rack (it comes after plural noun which is end in s)

2. Apostrophe is used for showing omision or contraction

- I'm = I am

- I'll  = I will

- Don't = do not

- Didn't = did not

- It's = It is or It has 

- etc

possession and abbrieviations

Frogs' legs            It's a car                       shan't= shall not

nice comprehensive answer by ismi puji......... agreed

it is used to show possessive case in grammar