Each letter designation represents a doubling of total impulse (thrust). A "C" motor has about 10 newton-seconds of thrust; a "D" motor has 20ns, etc.
Larger motors continue this pattern. For "model rocketry", in addition to the ABCD motors, there are also E, F and G class motors. An E motor can have anything from 21ns to 40ns. The Estes E falls somewhere in the middle. An F motor can have between 41ns and 80ns; most Aerotech F's have close to the full
80ns. A G motor can have 81ns to 160ns. Aerotech's "Econojet" G motors are just slightly more impulse than an F, about 90ns if I remember correctly. The G40 and G80 motors have about 120ns.Above G is "high power" which includes motors from H to O. High power rocketry has special rules and regulations that aren't found in model rocketry, mainly the requirement that the user be certified by one of the national organizations (NAR and TRA). High power rockets also generally need a "waiver" from the FAA, which is one reason why most people fly with a local club.
With the larger motors, you'll see more instances where the motor doesn't have the full impulse for its class.
Anything over an "O" is "amateur" or "experimental rocketry" and has a different set of rules.
Estes motors all use old-fashioned "black powder" propellent. There used to be some other companies that made BP motors as large a "H". Currently, the only commercially available motors bigger than an E motor use what is known as "composite" propellent, usually APCP (ammonium perchlorate composite propellent). This modern formula is safer and more efficient than BP propellent. A composite D motor could be as small as an Estes C motor.