WinDirStat is a port of KDirStat for Linux. It's lightweight, small (650kb installer), fast, portable (as a standalone .exe file), and works on multiple versions of Windows. Besides showing folders and percentages (for the entire disk or any subset of folders), it also displays an (optional) graphical usage map.
Environment variables hold values related to the current environment, like the Operating System or user sessions.
One of the most well-known is called PATH on Windows, Linux and Mac OS X. It specifies the directories in which executable programs* are located on the machine that can be started without knowing and typing the whole path to the file on the command line. (Or in Windows, the Run dialog in the Start Menu or Win+R).