The kind of sampling enTrap is talking about is the act of taking pre-recorded snippets of exiting music, usually riffs or grooves, and using them as hits or loops in another composition. I guess that some might use the term "sampler" to talk about an indivdual that works this way. But I think you are talking about "sampler" as in the Emu Emulator or the various Akai samplers.
A sampler is the device used to make the samples, as well as manipulate and play them back as a MIDI instrument.
A typical sampler gives you an environment in which you assign MIDI note values to the audio data, manipulate the attack-sustain-decay-release envelope of the sample, trim and possibly loop the audio (so that, if you hold the key down, the note either decays to silence or keeps sustaining by smoothly repeating the last part of the sound) add effects, blend the sound with other sounds, and otherwise tweak and bludgeon the original data. When you are done, you save the data in whatever sample format your sampler supports. That sound (or more typically, a set of sounds, like two octaves of French horn saved as a French horn sound set) can then be played back triggered by any MIDI controller from the sampler, or any other sampler or sample-playback synth that supports the file format it's been saved to.
Typically samplers are used to create MIDI instrument sound sets based on the actual recorded sound of individual notes from a real instrument, but anything you can record can serve as raw material (speech, noise, animal sounds, belches, etc.).