A PCB (Printed Circuit Board) is a board used to electrically connect and mechanically support components using pads, conductive tracks and several other features etched from sheets of copper laminated onto non-conductive substrates. There are various types of PCBs like single sided, double sided or multilayer PCB.
On the other hand, PCBA (Printed Circuit Board Assembly ) is the board obtained after all printing solder paste on the PCB and then mounting various components like resistors, ICs (Integrated Circuits), capacitors and any other components like transformers depending on the application and desired characteristics of the board. A PCBA usually undergoes reflow furnace heating to establish a mechanical connection between the PCB and the components.
In one word, PCB refers to a blank Printed Circuit Board, while PCBA (Printed Circuit Board Assembly) refers to a PCB mounted with components.