As far I understand them, Guy means:

A young person boy or girl

Singular = Guy

Plural = Guys

You can use it when you meet a group of friends and you want to say "hi" to everybody, you can say:

Hi Guys!

How are you guys doing?

Folk means a group of people in general, doesn't matter age or gender but sometimes very specified.

example:

Country music is the folk's music in texas.

If you live with mum and dad you can say: I live with my folks.

Dude is a word (slang) to refer to a Man or Guy, it is used as a friendly form in most of times.

Dude is a very american term and if I am not wrong it was created in New Your in 1880... or something like that.

You can say: Hey dude, it's my seat!

Hey dude, can you help me?

Thanks dude.

Cool dudes.

I was on the train with some dudes.

I hope everything is clear for you, english is my second language and I'm still learning...

Cheers.