When purchasing a motor the KV rating is almost always the most front and center spec other than the motor's size.
KV, not to be confused with the abbreviation for kilovolt, is measured by the number of revolutions per minute (rpm) that a motor turns when 1V (one volt) is applied with no load attached to that motor.
So basically (KV Rating) x (Battery Voltage) = Motor's RPM.
So if you are using a 180KV motor and a 10s3p battery your equation would look like this:
180KV x 42v = 7560RPM
Once the RPM is calculated, it can be used to calculate top speed and help guide your decisions on gearing ratio.
Here's the simple rule of thumb we use:
Higher KV Rating: Lower torque, Higher top speed.
Lower KV Rating: Higher torque, Lower top speed.
There really is more to it than just that, any brushless motor veteran may have their own opinion on that rule of thumb, but considering the sizes and types of motors we use in the electric skateboard community, this rule of thumb works for our purposes.