TOP is the top line layer (some factories call it L1)
BOT is the bottom line layer (some factories call it L2)
BOARD is the form layer of the product
GTS Top Soldermask Layer (some call it Top Window Layer)
GBS Bottom Soldermask Layer (some call it Bottom Window Layer)
GTO top side character layer
GBO bottom side character layer
DRL is drilled holes
ROU is milled edge file (also called gong tape)
If you're making a soft board there are also filler layers, bending zones, and so on.
Also if you are just learning CAM, we recommend learning genflex, this software is much more powerful than CAM350!