Steem and SBD ate both cryptocurrency like Bitcoin.. You can buy or sell just like Bitcoin.. BUT you can mine Bitcoin using highly specialised hardware and burn lots of electricity. But mining Steem or SBD doesn't need hardware but through upvotes.

Now there's a twist.. not all upvotes are same.. someone who converts their Steem into Steem Power will have more values in their upvotes. The more Steem Power you have the more influence your upvotes will be.