Together the subject of soils and rocks relevant to drilling is called 'geology' and the subject of water 'groundwater' or 'hydro-geology'. Being able to deal with the many different in soils and rocks and knowing where to find the water is how a driller really proves his skill.
The manual written by Arjen van der Wal 'UNDERSTANDING GROUNDWATER & WELLS in manual drilling' and hosted here along with a french version provides an excellent summary of the basic knowledge of soils, rocks and water a driller needs.
THIS PAGE NEEDS A LOT OF ADDITIONS, PLEASE ADD YOUR METHODS, TOOLS AND EXPERIENCES WITH THE DIFFERENT SOILS BELOW