one of the most wanted sounds in electronic music, rivaling the tr-808, tr-909 and several moog
synths. It was supposed to sound like a real bass guitar, which it totally sucked at. so roland, the manufacturer, stopped making them pretty quickly, so they're kinda rare. after that, house Djs in the 80's started messing around with them (Dj peirre?) and found that you could make some pretty wild sounds with them. 303s eventually came to form their own style of music Acid, where the 303 is in basically every song, or at least another synth trying to sound like a 303.
the 303 is actually a very simple synth, considering it only has one oscillator, only two waveform options (saw, square) and a (wicked) lowpass filter. so basically the filter is the best thing about it. it also has a built in sequencer, which allows you to program your bassline into it, then hit play and tweak the filter while it plays, making a freaking awesome sound! people also put effects on it, like phasor, distortion, and delay.