OK, this isn't a software analog synth emulator - all it does is allow you to alter the parameters on a fixed XG module (such as the DB50XG which costs under £100), but it is still allowing you to work with sounds in real-time and I will use it to give an idea of the synthesis-type facilities available on sound modules.
It is possible to layer several sounds together (like multiple oscillators, but you can use waveforms more complex than just sine/saw/pulse). Each of these 'oscillators' has a resonant low-pass filter, amplitide and pitch envelopes, reverb, chorus, LFO, variation (including distortion, overdrive, amp sim and auto pan), etc. Some other modules (e.g. Roland JV1080/2080, Korg TR-Rack) have even more extensive editing facilities including filter envelopes, selectable LFO waves, LFO-midi sync. The main differences between these and analog-type synths are the sound of the low-pass filter and the lack of features suck as FM and oscillator synchronisation.