Wavetable synthesis is a sound technology that uses samples of real instruments to create more realistic music playback. If a MIDI file calls for say a trombone to play a C note, the computer (or MIDI instrument) accesses the wavetable and chooses the appropriate note, pitch, etc. It's hard to imagine that this works and can sound good, but it does (with varying results depending on the quality of the wavetable, the hardware, etc.) Wavetable synthesis is, in most cases, much better than FM synthesis, its predecessor.
A method of sound synthesis in which waveforms are generated by loading their characteristics from a special set of parameters stored in a lookup table in computer memory. Advanced wavetable synthesizers are able to crossfade between different waveforms while notes are sounding, which can produce very complex sounds. The resulting complex waveforms are often further modified by other filtering techniques and envelope generators.