lumber "Codes"

I was looking in a lumber website, and as it turns out I’m more ignorant than i originally thought it was. I came across some notations that I have no clue what they mean. When they were talking about the thickness of the lumber instead of an actual inch figure they listed them as 4/4, 8/4 etc… do those x/4 corresponds to quarters of an inch? it also mentioned the specific gravity of each kind of wood, and finally the pricing used an FAS acronym. Anyone care to help me so when I walk into a lumberyard and ask for something I don’t look like a complete fool?... I’ll settle for looking like a partial fool… thanks!!