On earth there are two methods of propagation i.e. ground wave and sky wave.
In free space propagation occurs in the straight line or line of sight wave.
Below let us compare the difference between ground wave and sky wave propagation.
Ground wave propagation:
Ground waves exists between below 2MHz and covers ELF, SLF, ULF, VLF, LF and MF frequency bands.
The signal exists in proximity to the earth surface, it follows contour of earth and hence it can propagate considerable distances.
At lower frequencies, interference occurs due to atmospheric noise only, here trans it power is sufficient and hence maximum range of about 5000 miles can be achieved.
Optimum antenna size is about $\lambda$/2 , As the distance from transmitter increases received signal strength decreases and follows exponential curve.
They are also known as surface waves.
Sky wave propagation:
Sky wave exists between 2MHz to 30MHz and covers ELF, SLF, ULF, LF, MF and HF frequency bands.
It exists in the sky and depends on reflective characteristics of ionosphere layer, the signal reflected from ionized layer of the atmosphere.
In this propagation mode, signal can travel number of hops, back and forth, the example of this type is SW radio.
During right time, ionosphere layer is closer to the earth surface, it travels at a flatter angle, it travels larger distances there are more skip zones which results into no reception.
During the daytime, ionosphere layer is away from earth surface, it travels with an angle it travels smaller distances.