GUI
Tcl/Tk Graphical User Interface (GUI)
The Tcl/Tk GUI based is the classical GRASS user interface in the versions GRASS 4.2-6.3. For details, see the GIS Manager user manual and these screenshots.
wxPython GUI
GRASS's Tcl/Tk graphical user interface is currently undergoing a complete rewrite using wxPython. It is currently available for testing in GRASS 6.3 and will be the default GUI in GRASS 6.4 and GRASS 7+. See also these screenshots.
Command line
Not really a GUI, but loved by advanced GRASS users: the command line interface. Worth learning it!
External GUIs: QGIS (Quantum GIS)
The QGIS software gives access to key GRASS modules through a user-friendly plugin toolbox. QGIS can view GRASS maps directly alongside GIS data of many other formats. See also these screenshots.