Like most things in this hobby, motors come with a whole host of stats and acronyms.
The first thing to look for is the size, described by 4 numbers. The first two denote the diameter, the second two give you the height of the stator (the windings) inside. So, for example, a 2306 motor is 23mm in diameter & and has a 6mm tall stator. Bigger motors have more low-end power but they are harder on batteries.
The other key stat is the KV rating. It describes the number of times the motor will spin per minute per volt applied to it. So a 2400KV motor will spin at 2400 RPM per volt, so at full throttle, on a typical miniquad, it's spinning at nearly 40,000 RPM!
We go in to more detail in our Intro to FPV series here.