PCB is the acronym to Printed Circuit Board. A PCB is a carrier for the circuit's components and their electrical connections.
Most are made from a version of fiberglass and epoxy resin called FR-4. These boards are usually green, but can be any color. PCBs are typical 63 mils (1.6 mm) thick (1 mil = 1/1000"). Other thicknesses used are 39 mils (1 mm) and 31 mils (0.8 mm). PCBs can be single layer (traces on one side), two layers (traces on both sides), or multilayer (traces (or planes) on both sides and internal layers).