a monoski is a big ski, wide enough for two feet and shorter than a normal pair of ski.
It is different from a snowboard, because on a monoski your feet are not sidewise: they point straight ahead - and you are wearing normal ski boots. To visualize your skiing position on a monoski, imagine that you are doing your tightest traditional skiing: suddenly your two trusty skis fuse together ! Aaaaigh ! No longer you cannot open your feet a little bit !
Monoskis work only for very good traditional skiers. Anybody else will be left with two very large bruises on his hips, and wet clothes.