Steem Power gives voting power, so the more SteemPower you have, the more value it has when you upvote or downvote things; however, SteemPower is non transferrable . So you won't be able to send it to someone else or sell it like you could Steem. You can Power Up to turn Steem into SteemPower (this is instantaneous), and you can Power Down to turn SteemPower back into Steem (but this takes a long time).
When you power down it'll take 2 years to get it all. Steemit pays 90% extra reward for steem power. If I am not wrong, for example if you have 100 steem in steem power you could get around 190 steem after a year. So basically, if you power down after a year then 190 would be divided by 104 weeks(2 years) and you will get the equal share every week. So you have to wait for 2 years to get back all 190 steem.
Hope this answers your questions :)