Apple Mac OSX computers are now powered by Intel processors and thus you can run most software programs on a Mac that were written for a Windows PC because the Mac hardware is now the same as a PC.

Boot Camp Tutorial: Install Windows XP on Mac

With Apple Boot Camp, you can partition the Mac hard drive and install Windows on a new partition alongside OSX. The two operating systems then co-exist on your hard disk, allowing you to dual boot but you still need to reboot your Mac to switch from one to the other.

If that sounds limiting, PCW has listed some Bootcamp alternatives:

Parallels Desktop, VMWare Fusion and Sun Virtualbox (free) all allow you to create virtual machines (VMs) on your Mac hard disk, onto which you can install a complete operating system, such as Windows or even Linux (Linux is not officially supported by Boot Camp).

Also see: VMWare Fusion vs Parallels Desktop vs Apple Boot Camp 

The VM then runs within a window on the main Mac desktop, just like any ordinary Mac application, except that it’s running an entire operating system rather than a single application. This means you can see the standard Windows desktop running within a window on the Mac desktop, and can launch any Windows program within that VM.