Basically it's subtractive synthesis with some bells and whistles.
You have three oscillators that provide basic waveforms; also some FM and wavetables if desired.
Two filters that can work either parallel or serial.
Various modulators: two LFOs and three Envelopes (all three slightly different); and of course the vector envelope.
A few basic effects, but no arpeggiator or sequencer.
For many things the ES2 is my "weapon of choice"; Alchemy is too complex for most of my needs and Retro Synth often doesn't do it for me (except the Wavetable part which is in my opinion way better than the one in the ES2).