Steem is the fundamental unit of account on the Steem blockchain.
All other units (Steem dollars and Steem power) derive their value from the value of Steem.
There is no need to hold on to Steem in its cryptocurrency form, it should be used either to buy Steem power, Steem dollars or be converted to Bitcoins.
Steem increases in supply 100% each year. Steem was first published as a whitepaper in March 2016.
The currency and platform were in Beta until July 4th 2016. Steem focusses on Social dapps aimed at decentralising tradition social media platforms.