What is the difference between that and which? When should they be used?

http://www.kentlaw.edu/academics/lrw/grinker/LwtaThat_Versus_Which.htm

"That" is for specifying an item in a group. For example, "the computer that is on your desk". From the set of all computers, the one that is on your desk. "The money that came from your wallet". Which can be used in a question (not statement for the same purpose. "Which computer? The computer that came from your desk". You can never use which in the place of that for such a situation; "The computer which came from the store" is always wrong. Who vs. That: "The man who owns the computer store" vs. "The computers that come from the computer store". People vs. things. Do people agree or disagree?

Thank you Kerm and Ultimate Dev'r. This is an off topic question but what are the banner things that many users have on the bottom?

That's your signature area. We have chosen to put images about sites and projects that we care about in ours.

Some of us use that area as an actual signature.

I often have troubles with these/those two words. X.x I tend to have more troubles figuring out when to use these instead of those and when to use this instead of that, too.