Trimming is setting the 'control throws' (the range of motion over which a servo responds to an RC control input) of an airplane.
 Mechanical Trim
 Electronic Trim
Added complexity is created by the fact that one can set servo controls manually in the transmitter/receiver boards. In all but the cheapest transmitters, there are buttons available to trim a plane in flight for new stick zero points and modified sensitivity, so one can find the control input that flies well and then zero that as the default level.
'Expos', short for exponential curves, involve electronic trimming that gives smooth transitioning from precision control near the stick's zero point, to very rapid response for a small input at the extremes of the stick's range of motion. This provides a more natural interface; Someone who's pulling up rapidly usually doesn't care that his elevator is at 100% of extension instead of 98%, but someone flying a well-trimmed glider can control his whole plane, albeit in very wide curves, by going from 0% extension to 2% extension.
 Dual Rates
Dual rates allow a toggle switch on the transmitter to change the entire electronic trim configuration to one more suitable for another task, so the user may have one for landing and one for aerobatics.
Mixing is a function on moderate-complexity transmitters / receivers which allows for multiple different servo outputs to be modified by one stick's input simultaneously, and often in nonlinear ways. This is required functionality for most flying wings, which typically use 'Elevon mixing' to independently control two rear-wing control surfaces, which provide all control input in various combinations.
 PID Tuning
PID tuning is the performance tuning counterpart to the above in an autopilot control loop. Typically this involves setting short, medium, and long-duration responses to being off the desired axis of flight. PID tuning is very important to making quadrotors fly, and having settings that already work is one of the benefits of buying a premade system.