There's the obvious difference in improvising within a set framework, like 12 bar bluesy stuff, which can be very open to showing-off, and a lack of emotion, and total free-improvisation where none of the musicians have any preconceived ideas about where the piece is going. I favour the latter, and particularly like some of Lol Coxhill's stuff. He has recorded things with John Cavanagh doing electronics that sound lovely. Not being a muso I'd steer clear of the framework things, as I'd make too many errors, but totally free stuff allows a poor musician like myself to join in (no doubt some of you out there will think this would be a bad thing!)