A Sampler is an empty box which you can fill with your own sounds.
You can get cheap zoom samplers with a few pads on which you use to trigger your samples, they have limited storage and few functions. They are mainly used by DJ's for triggering phrases. They cost around £200.
At the top of the range, you have Akai, Yamaha and Emu. These are big rack mounted machines which you can attach hard disks, Zip disks and cd burners to. They have many options and allow you to hook up a keboard and computer, so that you can play them like you would a synth. You can fill these with whatever sounds you can think have. Most can have upto 128Meg RAM which means you can hold around 6 minutes of sound.
There are also Akai MPC2000 boxes, these are expensive samplers which are a cross between zoom samplers and hi end Akai samplers. Not quite, but it is a box with trigger pads on it.
Beat boxes come in digital and Analogue.
Old School hip hop and techno was written on Roland 808/909 drum machines, these are analogue and very expensive collectors items. There are modern copies of these boxes, a good one being the DrumStation. These are analogue machines, they give you that Phat stomp, even phatter when put through a compressor. Drum Stations cost around £300. Analogue drum sounds are modeled from sine waves and sound synthetic.
Digital drum machines contain samples of real drum hits and sometimes analogue drum boxes. They are basically boxes with drum samples in. Everything you can do on a digital drum machine you can do on a good sampler. The good thing about digital drum machines is, that they are there ready to use, no messing around.
If you buy one item, buy a good sampler. You can use it to do anything then, fill it with any sounds you like. If you don't mind a bit of work before hand. Plus, you can use it for you Bass, strings, organs and vocals.