In short:
steem is like having an assent in an decentralized platform.
Steem is inflated 100% per year. From that round about 90% goes to steem power holders 5% goes to curators, 5% converted to steem dollars as post reward.
(not mentioning the miner / witness reward)

steem power is steem that is ¨trapped¨ for at max two years (in average 1 year) . Steem power equals the power of your voting influence, urrently mainly used for distributing the post reward and electing the witnesses that take are of the blockchain consensus.

Steem dollars are pegged to the dollar and should in the long run give a stable currency to do commerce. The witnesses decide on the interest on steem dollars, currently it is 10% per year. Steem dollars can be exchanged to steem equal to the average steem price of one week. This conversion takes one week.

That it, have fun!