Basics: A microbiologcal organism/s is added to a food source for those organism/s. The process is usually started with good sanitation and the cultured medium is controled using air restriction, salt, vinegar or other forms of pH modification. Duration of fermentation is dependent on process and planned storage.
If you wish to try a fermentation I would recommend doing yoghurt first. Then start wherever you wish, Kefir, mead,
grains etc etc.
A book for simple introduction:Nourishing Traditions:Sally Fallon has many simple recipes which once the basics are understood are actually the same principle all the way.
The book by Sandor Katz backing the site mentioned above also has a simple entre. Bill Mollison's book is excellent. There is also an FAO website. CZ nourishing tradtions forum, Diet and Nutrition forum etc. Lots of materials and ideas are about.
Once you are have started fermenting the possibilities are only limted by your imagination.