SHCS | Socket Head Cap Screw: machine screws with hex heads in a 'socket' - i.e. the head is the socket, not the driver.
FHCS | Flat Head Cap Screw: similar to SHCS but have triangular heads, used when making fastener surfaces flush.
BHCS | Button Head Cap Screw: rounded head socket heads !
PLSTCF | Plastic Thread-Forming Screws: have triangular lobed threads and torx drive caps, wonderful in plastics as the beefy threads mean no inserts required.