DarWINE and CrossOver are Windows emulators that you can install Windows applications on. These do not require a Windows OS license.
With the other options -- Boot Camp, Parallels, VMWare and VirtualBox -- you install the full-blown Windows OS on the Mac, either in a separate boot partition, or on a virtual machine running under OS X. For these, you will need a legitimate Windows license to install the Windows OS on the Mac.
Which kinds of Windows apps are you interested in running? Some of them run quite well in DarWINE and CrossOver, and others don't.