x% chocolate means cacao + cacao butter. The more cacao there is the more bitter it will be. Bitter or dark chocolate means there is cacao and cacao butter plus some other things such as lecithin, it can even go as high at 100% at which point all you have is cacao + cacao butter. White chocolate simply doesn't have cacao, but only cacao butter + a bunch of sugar which is why it is so sweet. The order the ingredients are written is in descending order so pay attention when you buy chocolate. For example, there is 55% dark chocolate with more sugar than cacao or cacao butter.
You can use any chocolate you like, it is a matter of taste, just keep in mind you'll probably have to alter the recipe a bit.
The above answers were great regarding the chocolate.
Whipping the cream allows for your chocolate mousse to be airy. Without whipped cream, your mousse will just be a flat liquid. Once your cream is whipped, you can add incorporate your chocolate mixture and/or gelatin to stabilize it.