/diy/ - Do It Yourself

Can one of you kind gentlemen please give me a rundown on machine screws?
Identifying the head is easy enough but how do you identity the thread?
If I have an unknown screw or an object with a missing screw how do I know what screw type to order?

I assume perhaps the easiest way is to have a selection known of screws you can compare the screw to / try to fit in the hole, but what screws should I acquire for this purpose? Are there only a handful of different thread types that are used in commercial products?