Here we go:
F-Head: one valve in the head and the other in the block.
Hemi head: Hemispherical (cupped) head
I-Head: both valves in the head, directly over the cylinder
L-Head: both valves beside the cyinder in the block. (no rocker arms)
Valve-in-Head, Overhead Valve: same thing as I-head
Hemi was all the rage back in the early days because the hemispherical head promotes better combustion, however i think today 4 valves per cylinder, made possible by an overhead cam, is the more popular option cause you can get more air in and out of the cylinder faster.
L-head engines are generally smaller, lawnmower type engines like this one i took a picture of when it quit.