Types of comma, and rules surrounding them

The isolating/bracketing commas:

The most common type of commas and they are used to show a weak interruption in the sentence, something we could put in brackets hence the name. They usually occur in pairs although sometimes one of them won’t show.

He was, I thought at the time, a decent enough person.
I never realised, however, how much effort it will take to go to University.
Alfred Charles Gimson, born in 1917, was an English phonetician.

The easiest way to check whether you need commas is to imagine a given sentence without the interruption you’re trying to mark. If it still makes sense – commas should be there e.g. He was a decent enough person is still a sentence even after removing I thought at the time.

A listing comma:

It can be replaced by and or or:

I like eating bananas, watermelons and kiwis. / I like eating bananas and watermelons and kiwis.
I will go to Italy, Spain or Portugal. / I will go to Italy or Spain or Portugal.

A joining comma:

It is used to join two complete sentences into one and is always followed by and, or, but, while and yet.

I went to college, and my husband did as well.
I must have forgotten about my homework, or there was no homework at all.
I loved him, but he turned out to be a psychopath.
She was a good and loyal person, while his new girlfriend is a lying cheat.
The train was running, yet I felt I was stuck in the same place.

A gapping comma:

It is used when omitting words already used in a sentence in order to avoid repetition.

I dyed my hair black, and my sister, red.

Bear in mind the comma before and is a joining comma, whereas the one before red is a gapping comma (used instead of repeating has dyed her hair).