If you don't know what it is, you shouldn't buy one! Only buy stuff like that when you NEED it. Otherwise you'll only end up selling it and giving hardware a bad name blah.

Anyways I have one and I love it (needed it too )

A sampler can record and play back.....samples. Surprisingly.
There are generally two types of samplers, phrase samplers and "normal" samplers. Phrase samplers are meant to playback loops. They often have functions to automatically adjust the lengths of loops so that their speed matches the rest of your loops. Phrase samplers however are not meant to play melodic lines with a sample, for this you'd need a "normal" sampler. With this you can play samples over a keyboard, and usually apply filters and ADSR envelopes and LFO's and similar synth-ish functions.