OK. Here goes.
Bootcamp
Basically,
Bootcamp lets you install Windows on a separate partition running Win natively. The advantage: your Mac is now a PC. A Windows setup CD/DVD is required. Bootcamp creates a Windows driver CD for all your driver needs. Once that's done your Windows will work quite well. The disadvantage: 117,000 viruses. Spyware, malware and so on. You've got to treat it like you would any PC: with lots and lots of security software. Head over to Download.com for all your PC needs.
For BootCamp I strongly recommend
smcFanControl. Increase your fan speeds before booting into Windows. The way a Mac is designed does not allow you to tinker with the fan speeds when using the Redmond O/S. Gaming will warm your box up so keep it cool that way. When you return to the Mac OS slow the speeds down.
Parallels
Parallels is A-1 software (forgive me for sounding like a shill). It runs Windows software while in the Mac OS. And, if you created a BootCamp partition then running Windows software in Mac is a breeze (BootCamp will find your Windows partition and set everything up for you!). However, certain software may have issues. I cannot give a full list, but when I tried Homeworld (the game) under a Parallels virtual box, it was a NoGo. But it worked 100% under BootCamp. I didn't try running Homeworld in Mac O/S once it was installed in XP though.
Tip: After you install Parallels your Windows boot process will go "wrong", sort of. XP will boot, but there will be a list of two different XPs to choose from. One is a standard XP, the other a Parallels enhanced version. The Parallels Enhanced version is default and doesn't work. You've got to be quick on the draw and move your cursor keys when the choices appear. This stops the timer and lets you select the standard version. Once you're in Windows you can tweak the time it takes for that menu to remain there. It is set for 0 seconds... you may want to extend that to 5 or so. How? I could tell you, but it would take all the fun out of your own search.
Conclusion
Parallels is the only software I've ever purchased for my Mac. Everything else is either Freeware, Shareware without nags or OpenSource. But Parallels rather impressed me.