Both Parallels and VMware Fusion do the same thing, along with
Virtual Box (which is free). They allow you to run your installation of Windows in a virtual machine alongside OSX. You can also use them to install other OSes in a virtual machine such as Ubuntu, another version of Windows, etc. Both Parallels and Fusion have trial versions, and you can try out their features for yourself. The differences are mostly personal preferences.
Boot Camp allows you to boot natively into your installation of Windows. Parallels and Fusion will read the Boot Camp partition, if you install this way.
For more complete information, you can refer to the Guide:
Booting Windows on a Mac
Woof, Woof - Dawg