S2S- means surfaced 2 sides
S4S- means surfaced 4 sides
1/8- means 1/8 inch thick
Hardwoods sold in the rough go by the quarter scale.
4/4- means 1" thick
5/4- means 1 1/4"
6/4- means 1 1/2"
8/4- means 2"
16/4- means 4"
Most sawmills saw about 1/8" over target so the 4/4 will be about 1 1/8" thick off the mill, etc.,etc.
Here's a link that has many wood suppliers.
http://www.woodfinder.com/