Motors are generally rated by their size usually with two numbers such as 2205. First two numbers indicate wider motors in diameter, second two numbers indicate taller motors. Increasing or decreasing each number has a direct effect on its speed and torque.
Smaller motors tend to have lower torque but spin much faster to compensate for that. Smaller motors are also inefficient in relation to their larger counterpart. In example, a 2205 motor could consume 50 amperes at 16 volts to generate 1500 grams of lift, whereas a bigger 3620 motor could achieve the same lift by consuming only 15 amperes at the same voltage. Of course, the efficiency of the bigger motors comes with a cost, which is weight.
Another rating of the motors is the kV, which doesn't stand for kilovolts. Instead this is a rating which indicates how many revolutions per minute the motor spins for each given volt. In example, a 1000kV motor will spin 12.600 times per minute when powered by a 3 cell, 12.6 volt battery. Generally lower kV motors work best with bigger propellers and this setup is more efficient.
The motors used in UAV's are mostly brushless tri-phase motors requiring their own computer to help them spin. These motors are completely immune to water so they can be submerged and still work perfectly fine.