Commas

A comma (,) can be used to separate things in a list.

I like to eat apples, grapes, pears and oranges.

A comma can also be used to show a break in a sentence. It can separate different parts of a sentence and show you where you should pause when reading.

I like swimming, but I love cycling!

With a shake, the dog dried itself off.

Anyway, I like chocolate too.

A comma can make the meaning of a sentence clear and change what you are trying to say.

Let’s eat Grandma.

Let’s eat, Grandma.

http://www.oxforddictionaries.com/words/punctuation

http://www.bbc.co.uk/skillswise/topic-group/sentence-grammar