Yo Rightly.
Vicki pretty much nailed it with her reply to your questions.
Look upon Soundcloud as a hosting/sharing platform and a community similar to this forum,but a bit more widely used. As with here,you get out what you put in. The more you listen to the music of others,the more they listen to you. It's one way of getting your music out there and also good for a backup for your music,if/when your computer dies.
There is no way currently to sell music through Soundcloud,but you can make folk aware of your music and point them to where it is for sale...such as a link to Bandcamp. Some also use Facebook as a medium to plug a sale,personally I've never found it to be much use for actual plays or sales, beyond a few folk who may know you. Perhaps I don't have a big enough profile on Facebook,but it wears me out after a few minutes!
There are loads of other hosting platforms such as Spotify,Deezer,YouTube,iTunes...etc. All these can get your music out there too. Others seem to have more success with Spotify than I do. I get hardly any plays at all,maybe I'm not using it right?
Band camp take something like 20 or 25% cut for everything you sell,which is probably reasonable enough.
So to sum up,places like Soundcloud can get your music played and Bandcamp will allow you to sell it.