Jim-
IP stands for Internet Protocol. Basically, it’s kind of like a street address for the internet.
If you have ever gone to a website, and seen something like http://68.122.148.15, the numbers are an IP address. They tell other computers where to look for a website. All other website names (i.e. http://www.chiffandfipple.com), have an IP address assigned to them. Other computers have databases whose only job is to look up the “normal” english type name, and get what IP address it belongs to, and send you there.
The flap is that when you have a service such as Cox, Qwest, AOL, etc. and don’t have your “own” IP address, you are generally sharing one with a group of other users (Known as a “pool” generally), so it looks like the same thing to a computer.
Where this causes trouble, is say you are using Cox, and a spammer in the next county is using Cox. Spammer does his thing, people complain, and the IP address is listed as being “blocked”. Not only for the spammer, but anyone else in that pool that might want to use it.
There are other ways, known as “spoofing”, where spammers can make their emails appear to be from another IP as well.
Hope that helped a bit…
Aodhan