The Windows environment is a leftover from the days of DOS, a place to store variables that describe various details about your system configuration. If an application wants to know where to store temporary files, for instance, it can check the Windows environment TEMP variable for the answer.
You can edit the environment within Windows, but it's not exactly easy. In Vista, for instance, you'd right-click Computer, select Properties > Advanced System Settings > Environment Variables, choose a variable, click Edit, and make the change you need in a cramped text box.
Rapid Environment Editor makes the process considerably easier, by displaying all your system and user variables in an easy-to-use GUI. It's able to add variables and values, edit existing settings, copy and paste them, and includes advanced features like the option to switch variable types or convert paths (long to short, or short to long). And if you make a mistake, don't worry - click Undo and try again.
Perhaps the most useful feature is the editor's automatic checking of paths and filenames. If you accidentally enter a non-existent path (or there's an environment variable left over from a program you've uninstalled), then there's no chance of missing the error - it'll be highlighted in red, immediately alerting you to the problem.
There are plenty of other interesting tools and options, though. If you're worried that you might accidentally delete or edit the wrong variable, for instance, that's not a problem - the program can save your current environment as a standard REG file, making it easy to restore (on this PC or any other) at a later date.