Some programmers are universal and will fit any vehicle. There are others that are particular for a certain model, engine, make and year of the vehicle. These tuners have chips inside carrying information related to a variety of vehicles and their engines.
The programmer allows you to modify the air/fuel ratio and other parameters to boost the performance of your ride. These tuners have the data to check how far your vehicle can be pushed to take full advantage of the capped parameters.
Signals are sent through the programmer to override the factory ones to boost power. Such tuners allow you to customize the tunings and set your own parameters. You can either choose from a pre-loaded tuning option or set your own.