What is F2.0, f 3.4mm stand for, how do I choose lenses from these numbers?

F stand for aperture of the lenses is on F stop 2:1 and f 3.4mm stands for the focal length of the lenses is 3.4mm
Lens with F 2.0 and f 3.4~4 is most economical to build, hence much cheap. It is widely used in single-board camera. Lens with F 2.0 aperture will collect about half light of human eye. Lens with f 3.4mm has about 60 degree view angel on 1/4" CCDs and 90 degree on 1/3" CCDs, which is pretty closed to human viewing angle. Human with two eyes can cover more view angel as an ingenious design of God. It is generally in the range of 150 to 180 degree from people to people. However please keep in mind that F stop and f focal length is the basic spec of lens, yet it doesn't stand for the quality. A good lens with same F stop and focal length can be 100 times more expensive then a bad lenses in same spec.