Waxless skis with xc skiing have "a bit" different meaning then exact words they carry on :) With xc skiing you have 2 types of waxes (at least for classic technique). One is normal glide wax (front and back part of ski), the other is kick wax (middle part of ski under boot). So this "waxless" goes just for kick wax. With right pattern (for low end skis) or right preparation of base (also for race skis) you can avoid waxing with kick wax (for race skis it goes only for particular snow type and particular temperatures). Glide wax on the other side is still needed if you want to achieve better glide.
As far as wax difference goes... yes xc and alpine waxes (glide waxes of course) are different. Alpine waxes need to last only for 2-3km, but they need to work best at speed around 100km/h. Xc skiing waxes need to last up to 50+km, but they need to achieve best glide with speed between 20 and 30km/h. So for race purposes, they use totally different waxes for xc then they use for alpine. For normal people this doesn't really matter. Anything will do, and anything is better then nothing, so don't worry if you are using alpine waxes for your xc skis... just take care you avoid middle (kick) zone of your ski :)