BootCamp is installing Windows to a separate partition on the drive. If you want to run Windows, you have to restart the computer and tell it to boot into Windows. It is 100% native, but Apple's drivers aren't always all that great in Windows (what's the motivation?) and there's a BIG battery hit.
Fusion/ Parallels is virtualization. It lets you install Windows into a virtual machine. You can then switch between OS X and Windows like you would switch between firefox and chrome. As someone else pointed out, both of them will actually let you use a bootcamped install of Windows as your VM (virtual machine), then you don't have to juggle files and settings in as many places.
Back when I had both OSes on my MacBook, I usually preferred Fusion, they were usually behind Parallels in terms of bells & whistles, but when they did roll the stuff out it was usually a little better implemented. Though, this was at least 5 years ago when the scene was still young.