You can build sequencers right in Max to control time/events firing off samplers, synths, effects, that you can also make right in Max.
Max also hosts VST and Audio Unit plugins - this way they can be treated like a modular building block just like other 'objects' in Max. Gen~ is a way to do optimized digital signal processing in Max that work at a single sample level and that do not need to be compiled in C.
I guess the best way to think about Max is as a huge modular machine where almost anything can be patched into anything. The beauty is its ability to finely control sounds and trigger them however you want - randomly, through probabilities, external data, conditional statements (ex. if X happens then do Z), etc. - whatever you can think up, you can do - (ex. I want this rhythmic and pitch structure to interpolate/modulate seamlessly to this new structure over the next 30 seconds).
Hope this helps :)