We have a bit on that in our tooling section: http://docs.carbide3d.com/support/#tooling-support
The community-maintained wiki has a bit more: https://www.shapeoko.com/wiki/index.php/Endmills#Shape
Basically, a square endmill will cut a flat bottomed slot / pocket, while a ball-nosed endmill will cut a U-shaped channel, or a pocket w/ a series of scallops.
A ball-nosed endmill can be used to do 3D contouring though, with the CAM program planning the paths so as to get the rounded end as close into the shape to cut it as is possible.
and you may find the Glossary of interest as well: https://www.shapeoko.com/wiki/index.php/Glossary