Comma:

It is a punctuation mark(,)used to:

to separate words in a list:

though they are often omitted before a bouquet of red, pink and white rose tea, coffee, milk or hot chocolate

to separate phrases or clauses:

if you keep calm, take your time, concentrate and think ahead, then you’re likely to pass your test.

After all the excitement of the party, the children soon fell asleep.

before and after a clause or phrase: that gives additional, but not essential, information about the noun it follows:

the Pennines Hills, which are very popular with walkers, is situated between Lancashire and Yorkshire.

(do not use commas before and after a clause that defines the noun it follows). The hills that separate Lancashire from Yorkshire are called the Pennines.

to separate main clauses, especially long ones, linked by a conjunction such as and, as, but, for, or:

We had been looking forward to our holiday all year, but unfortunately, it rained every day.

to separate an introductory word or phrase, or an adverb or adverbial phrase that applies to the whole sentence, from the rest of the sentence:

Oh, so that’s where it was

As it happened, however, I never saw her again

By the way, did you hear about Sue’s car?

to separate a tag question from the rest of the sentence:

It was pretty expensive, isn’t it?

You live in Bristol, right?

before or after ‘he said’, etc. when writing down the conversation:

Come back soon, she said

before a short quotation:

Disraeli said, “Little things affect little minds’