All new Macs (ie. Those with Mac OS 10.5 "Leopard" installed) come with an application called Boot Camp.
Boot Camp allows you to repartition your hard drive and start the installation of another OS (Such as Windows). It will also burn a driver disc for you to get all your hardware like webcams, graphics cards, etc up and running.
Boot Camp is "real" dual booting. When you power on the computer, it will give you a choice of OS's to start. So you get the full hardware performance in your chosen OS. There is no emulation.
There are third party virtualization technologies out there (VM Ware, Parallels, etc) which allow you to install other OS's and run them from within Mac OS, so you have two OS's running at once. This has the benefit of not needing to reboot when you need to use another OS.
Personally, I prefer Boot Camp as I actually "feel" like I'm within Windows or Mac OS.