What does FR-4 mean?
FR means "Flame Retardant" and 4 means fiber glass epoxy. This term is used to describe the most popular material to manufacture circuit boards.
More info is availabe at EMTontheNET. Here is some of the detail:
FR4 laminate is the usual base material from which plated-through-hole and multilayer printed circuit boards are constructed. "FR" means Flame Retardant, and Type "4" indicates woven glass reinforced epoxy resin.
The laminate is constructed from glass fabric impregnated with epoxy resin (known as "pre-preg") and copper foil, which is commonly supplied in thicknesses of "half-ounce" (approx 18 microns) or "one-ounce" (approx 35 microns). Foil is generally formed by electrodeposition ("ED Foil"), with one surface electrochemically roughened to promote adhesion. A typical 1.6 mm rigid laminate is made by bonding together eight plies of heavyweight pre-preg, with half-ounce copper foil either side, in a hot hydraulic press.