The reliable number cruncher
GRASS GIS is powerfull free and open source software for performing spatial analysis. It consists of more than 500 modules for processing vector, raster, voxeland temporal data.
The roots of GRASS GIS are going back to the 1980s, but it is still at the bleeding edge of innovation in GIS technology.
It has been developed with efficiency in mind and is suitable for both small and big data problems. GRASS GIS runs on almost all kinds of hardware, from small laptop to HPC cluster and can serve as a desktop GIS with a modern graphical user interface as well as the backbone of a GIS infrastructure.
More than 200 AddOns can extend its functionaly and provide solutions for often more specific tasks.

Active GRASS GIS community 
Many people have contributed to improve the software. Its strength and success rely on an active development team and the feedback of a wide contributors community; both combine their efforts to make GRASS GIS easier, more useful and powerful to everybody.

Where is GRASS GIS used?
GRASS GIS is used in scientific applications, commercial settings and by public authorities all over the world. The software has shown strong potential for solving geospatial problems in numerous situations world-wide.

Features
GRASS GIS supports nearly all common GIS file formats through the use of the GDAL/OGR library
Raster analysis: map algebra, interpolation, mask...
3D raster (voxel) analysis: 3D map algebra,3D interpolation, 3D visualization...
Image processing: aerial/UAV image, satellite data,supervised/unsupervised/object classification...
DTM analysis: contour/surface generation, cost-path, slope-aspect analysis, hydrology
Vector analysis: topology, buffer, overlays, network analysis...
Temporal (4D) framework: support for time series big spatio-temporal environmental data
Point cloud analysis: LiDAR, interpolation...
Spatial statistics: correlation/covariance analysis, regression...
Geocoding: raster and vector maps
SQL-support: different database backends can be used (SQLite, PostgreSQL, ...)

Interfaces

GRASS GIS can be used through different interfaces:
the simplest for new user is the Graphical User Interface (GUI) with several powerful tools
power users use the text-based commandline interface (CLI)
there is a C API for programmers
for Python language there is a scripting library and an object-oriented Python API
also the statistical language R can access GRASS GIS functionality (or the other way around)
web interface through WPS servers
Furthermore, GRASS GIS provides many interfaces to other programs in related domains like geostatistics, databases, web map services and even interfaces to other GIS software exist.