Both chorus and unison are ways of fattening up a sound - of making it denser, however the way they achieve this is different.
A chorus effect is achieved by taking the original sound, delaying it slightly, modulating its pitch and panning it, then mixing this back with the original signal. The pitch and delay are then altered over time. This can be done numerous times, each adding another layer varying from the others in timing, pitch and panning. The interaction between the different voices and the original signal creates a beating effect. It is effectively simulating the effect of multiple separate voices ( a chorus of voices).
Where chorus simulates multiple voices using a single original voice, unison actually uses multiple voices, varying their pitch slightly to achieve a fatter sound. On a synthesiser this is sone by duplicating oscillators.