There are four major modes that the waves transmitted from a transmitter may follow to reach the destination and they are
(a) Surface wave propagation
(b) Space wave propagation
(c) Troposphere propagation
(d) Ionosphere propagation.
The first two propagation modes are grouped into ground wave propagation, but the behave differently enough for separate consideration.
Meaning of different terms in Figure :
TX = Transmitting antenna
RX = Receiving antenna
Path A = Ground wave propagation
Path B = Sky or ionosphere propagation
Path C = Space wave propagation