You can if you have a Verizon smartphone that is rooted and running WiFi tether. That's the only way. Verizon uses CDMA, ATT uses GSM, so they are not compatible networks.
If you're not too computer savvy, I would just forget it. You have to hack your phone and install stuff and no it's not a cord, it's over WiFi.
I wrote this for the Droid Forums before I was banned for defending the iPad over and over...there are approximately 100 steps to the process. Some of the links are probably outdated by now, but that just gives you an idea of the process if you're not familiar with doing it. It takes me about 5 minutes, but if you've never done it before and make mistakes it could take you a day.
Good question. People do it all the time on Craigslist and stuff for a fee. I could do it for you but you'd have to mail me the phone. There used to be a really simple site that broke it down to the very basics called rootyourdroid.info but they shut it down I think.
There are some apps on the Market that do it for you (or there used to be not sure about now) but you still need to know the basics.