I will try to simplify and summarize, hoping it will help you.
Steem is the base currency of the platform that can be transformed into Steem Power through Power Up and this allows us to increase the value and power of our account. It is worth about 0.40 cents on the dollar now.
SBD would be a stablecoin, its value should be tied to the dollar, however for several months now it has had a market price ranging between $5 and $7.