Steam Workshop is basically a platform for user-generated content, where you can upload your games and have others play them, rate them and comment on them. The games you upload can be works in progress or finished games, but they cannot be for sale as the nature of the Workshop is to offer user created content for free. You should also be aware that the games uploaded to the workshop cannot make use of extensions in any form as Steam consider them to be a security risk. You can have included files however, and you need not worry about whether your games will work on Mac OS X, Ubuntu (Linux) or Windows as that side of things is handled by Steam.