Now this is an old video now, it is a great example of how antennas work with the patterns an gains. Its dated because Circular Polarized antennas are not covered but still a great resource for learning the basics in simple terms.
FPV Antennas 101 - Polarization, Diversity & Gain Patterns (http://www.youtube.com/watch?v=cMIqlsDu9h8#ws)
Credit to Andyology
Bruce for RCModelreviews explanation of FPV antennas
FPV antenna gain and range explained (http://www.youtube.com/watch?v=Bx7gMUw1MGw#ws)