Noob question - template vs hack

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Hello everyone -

First, my congrats or thanks (whichever is appropriate) for the look of this site - I have been surfing through thousands of VB related pages at VB, SPF, VB.org and here, and your layout is very easy on the eyes.

I have a newbie question - I have someone doing the major construction on my forum, and she wanted me to tell her which hacks and template mods I wanted. I know that:

This site houses template mods and postbit mods, and,

VB.org houses hacks.

What I don't know is:

What's a postbit modification, and

What's the dif between a hack and a template mod?

Can someone 'splain that to me? Is one harder than the other?

Thanks!

Jeff