The clearest difference that you'll find is that a "Site Admin", does not have the manage site link. All the things that you can do on the backend such a add elements, add pages, change design, and ect... won't be accessible by Site Admin/Level 8.
As far as what they are allowed to do after that being said... In each of the sites "feature pages" (blogs, discussions, wiki, events ect..), you'll find an icon on the top right of each of those pages in the content areas. When in management mode, it will instead be a button that says "edit feature settings". When clicked, there will be an option to assign what level of access people need to have to moderate the individual feature.
