Several important characteristics of the performance of a motor are used commonly when describing it, both on these pages and in general hobby parlance. For illustration purposes, here is the thrust curve of an AeroTech G80 (original graph from the NAR data sheet).
Total impulse is the product of thrust times duration over the motor burn time, and is measured in Newton-seconds (Ns). This measures the total amount of momentum imparted to the rocket by the motor. The total impulse (along with such factors as rocket mass and air drag) determines how high the motor can propel your rocket. (Thanks to Greg Lyzenga for this description.)
The impulse class is a letter code assigned to a range of total impulse with each letter being twice as powerful as the previous. For example, the "AeroTech G80" shown above has a total impulse of 120Ns making it a 'G' class motor.
Average thrust is the average instantanous force the motor produces during its burn and is measured in Newtons (N). Note that because most thrust curves are not flat, most of the time the motor is not producing the average thrust. The average thrust tells you how heavy a rocket the motor can lift, although since different motors produce different shaped thrust curves, it can be misleading. (Average thrust is determined by dividing the total thrust over the official burn time—within the 5% threshold.*)
Maximum thrust is the maximum amount of force produced by the motor during its burn. Like average thrust, this is measured in Newtons (N). Generally, there is a small spike near the beginning which has the highest thrust, although the shape of the curve varies with the motor design and propellant mixture.
Burn time is the number of seconds for which the motor produces thrust. This tells you how long the motor will keep pushing your rocket. (Burn time is determined by chopping off the ends of the curve when the thrust is below 5% of the maximum.*)
The common name of the motor is formed from the total impulse, represented by the impulse class, and the average thrust. For example, the "AeroTech G80" shown above is a 'G' class motor and has an average thrust of 77.5N (rounded to 80 in this case).
Another interesting number is the specific impulse, or ISP. This number is the total impulse over the weight of the propellant, which gives you a measure of the motors efficiency for lifting since the higher this number is, the more impulse you get for the propellant's weight. Think of this number as the quality measure of the propellant formulation. Isp may be expressed in "seconds" and intuitively, it means something like "how many seconds this propellant can accelerate its initial mass at 1 gee". (Thanks to Robert J. Kelley for this description.)
The mechanical properties of the motor are also interesting for motor selection. Of course, the diameter tells you whether it will fit in your motor mount tube. The length gives you an idea of how much space you need in the rocket for the motor casing. The total weight is very helpful when testing your rocket's stability. When measuring the center of gravity, if you don't have the motor and casing handy, you can make up a dummy of the same weight using shot, dirt or whatever you have handy. See the CG/CP Relation article in INFOcentral Rocket Design section for more information on rocket stability.
Note that on this site, all statistics listed on the motor page are from the certification organization (as published on their web sites). For more information on these organizations, see the Certification page.
* Based on NFPA 1125:
This means that the letter for the motor class is based on the area under the whole curve,
but the value after the letter is based on the average thrust during the truncated burn time.