A souffle, is like a cake, but airier. While mousse, is a chilled pudding-like dessert. It can also be made into a cake, called mousse cake, where it is mixed between a layer (Or more) of biscuits,...
As far as my cooking experiences extend, a traditional mousse does not contain gelatin or any other artificial setting agents.
(The best chocolate mousse I know of doesn't even contain cream--Allow...