Cedega is based on Wine
, which implements the majority of the Win32 API on the Linux/Unix environment. Cedega, a commercial product, is especially targeted toward gamers.
Approaches such as VMWare are true hardware emulators,
creating an environment in which a true Windows installation can run as a "guest" operating-system in what appears to be a proper hardware
Both of these approaches are powerful within their appointed limits. They may, or may not, be a suitable substitute for having a licensed copy of Windows on a separate, bootable drive. It depends on what you use Windows for, how often you use it, and how well you find that your "mission critical" applications do (or do not) run on the alternatives.