CHANGES in GRASS GIS 8.x compared to GRASS GIS 7.x

Efforts have concentrated on making the user experience even better,
providing many new useful additional functionalities to modules and
further improving the graphical user interface. The new graphical user
interface with entirely rewritten startup sequence.

https://trac.osgeo.org/grass/wiki/Grass8/NewFeatures80

See list of releases with changelog at
https://trac.osgeo.org/grass/wiki/Release


CHANGES in GRASS GIS 7.0 compared to GRASS GIS 6.x

The graphical user interface based on wxPython (introduced in GRASS GIS 
6.4) has been enriched with many new features in order to make complex 
GIS operations available as simple as possible. The old Tcl/Tk based 
GUI has been dropped. Relevant new features are available in the core 
system, among the most important the  new Python interface to the C 
library. This new feature permits Python developers to create new 
modules in a simple way while at the same time creating powerful and 
fast modules. Furthermore, the vector library was particularly improved 
to make it faster and more efficient with support of  huge files. This 
required a  easy to manage topology format update including a new 
spatial index. Finally, there are a series of new modules to analyse 
raster and vector data; some of the already existing modules were 
improved and made faster (some even 1000 x faster). 

In total, more than 10,000 changes have been implemented in GRASS GIS 7.0.0
with respect to GRASS GIS 6.4.x

For details, please refer to this Web page:
http://trac.osgeo.org/grass/wiki/Grass7/NewFeatures