GRASS GIS for ArcGIS users
Projections
ArcGIS supports on-the-fly projection of spatial data while GRASS GIS considers this as a bad practice and requires user to have consistent projection for all the data entering the analysis.
Data, databases and file formats
In ArcGIS, users often have data in different directories on disk. GRASS GIS sets a specific system to organize the data. First, data must be in one directory called GRASS GIS database directory. You can have one or more of these directories on your disk. This directory contains GRASS Locations. All data in one Location have the same projection (coordinate system, datum). Location is a directory which contains GRASS Mapsets. Mapset contains raster and vector maps (layers) and other geospatial data.