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.