It's just a shitty monosynth.

The TB-303 is an analog monosynth released by Roland in the early '80s. It was originally made to emulate a bass player. A guitarist would purchase the TB-303 along with the TR-606 (a drum machine), and (so the theory goes) they would not need a band for any practice. As it turns out, it was particularly bad at emulating a bass player.

In the late 80's a techno track came out called "Acid Tracks" by Phuture. It was the first AcidHouse track to be made. Since that track, the Acid Explosion happened, and everybody wanted a TB-303. It is very difficult to emulate the sound of the TB-303, because of the way the slide and TheAccent work.

The x0xb0x is almost a complete replication of the TB-303s analog synth, and only diverges with respect to the PowerSupply and the DigitalSequencer. For more information on the x0xb0x and the TB-303, see Tb303x0xb0xDifferences.