-ing forms are used:
as the subject of a sentence:
Dancing is fun.
Reading is useful.
after prepositions:
I am thinking of reading a new book.
She is bored with doing the housework.
after certain verbs, e.g. love, hate, prefer:
I love dancing.
I hate swimming.
as part of continuous form:
They were cooking a cake.
My brother is reading a book.