QGIS (Quantum GIS) is a sister project to GRASS GIS. Our two projects share programmers, infrastructure, and code when we can.
It features an easy to use GUI interface which may be familiar to users of other common GIS software. Whereas GRASS GIS focuses more on data processing and analysis, QGIS focuses more on cartography and map making. They compliment each other well and you can even use the QGIS GRASS Toolbox to load in GRASS data directly, and run GRASS processing commands from the QGIS menus. Another option is Processing in QGIS which uses among other interfaces GRASS GIS as backend.
We are both founding members of the Open Source Geospatial Foundation (OSGeo)
New development in 2015: