I'm new here and probably least qualified to answer this but I'll take a stab at it as the general feeling I get is that people ignore posts when they get this old unless they're part of a series...

Steem power is a user's influence over how much their vote is worth. It's the sum of their steem and any delegated steem. Delegation is using one user's steem power to temporarily boost another user's steem power.

Steem power influences vote worth. That is, higher steem power votes will tell the system to pay out more than a lower steem power vote.

SBD is roughly a quantity of steem that is equal to a USD in value. 1 SBD is supposed to be tied to 1 USD, or so I gather. SBDs are like dollars, which is good because the value is pegged and stable.

Steem has variable worth. So if the value of steem goes up, your steem becomes more valuable, but the SBD is still worth the same. If steem goes down in value, your steem is worth less, but your SBD is still worth the same. SBD can accrue interest, but I haven't looked into that, yet.

The value is going to fluctuate over time because, er, markets? Speculation?

It occurs to me now that someone else probably already answered your questions elsewhere, but I feel compelled to respond anyway.