This list is NOT comprehensive. I will try to keep it as current and complete as possible.
Low Power Rockets: Rockets that generally weigh less tan 1500 grams (3.3 lbs.) and fly using a motor or motors ranging from 1/4A to D impulse. The motor or combination of motors must have a propellant weight of 125 grams or less, no more than 160 Newton-seconds of impulse and an average thrust of less than 80 newtons.
Mid-Power Rockets: Rockets that meet the all of the above weight and power criteria, however they are designed to fly using more powerful motors ranging from E to G impulse. Both Low and Mid-Power Rockets are considered to be "Model Rockets".
High Power Rockets: Though they may be scale replicas of actual rockets. High Power Rockets are NOT model rockets. Flying them requires a higher level of skill. Flyers must be certified in order to both purchase high power motors and fly high power rockets. The motors generally range from H to O impulse. Impulse ranges past O impulse exist, however they are out of the range used in sport rocketry.
Low and Mid-Power Rocket Manufacturers : The vendors on the left primarily manufacture model rockets (low and mid-power). However, both Sirius Rocketry and Hangar 11 manufacture a few high power kits and sells low to high power motors.. Hangar 11, eRockets and Sirius Rocketry sell high power kits by other manufacturers.
High Power Rocket Manufacturers: The vendors on the right manufacture high power rocket kits and some offer mid-power as well.