Steem Power(SP) is the token that gives users aka "Steemians" the capacity to influence on the Steem network, ie Steemit. As you participate in the Steemit platform creating quality content and voting on other fellow Steemian's work, you will be rewarded in SP. By having more SP, your voting influence will directly increase as well. Steem Power can also be converted from STEEM (What is STEEM?...Check out the next paragraph :D ) You can also purchase Steem Power in your wallet. If you hold aka HODL Steem Power you will also be rewarded a portion of the newly created Steem as more Steem Power...How you ask?....By voting and creating content on Steemit and other Steem platforms.

Steem (STEEM) is the token that can be utilized in a cryptocurrency exchange. For example you can trade STEEM for Bitcoin (BTC) and if you wish to USD as well (not directly). Steem can be converted to Steem Power, which will increase your influence on the Steemit platform. You can also purchase Steem in your wallet. Steem is a very volatile crypto that is based upon the open market. It doesn't really fit in the whole Steem world. It's primary use is to convert from your Steem cryptos (like SP and SBD) to Fiat(ex. USD) or other cryptocurrencies on the market and vice-versa.

Steem Dollars (SD or SBD) is a stable token that is valued at about $1.00 of STEEM. The value of SBD will remain the same price. If and when you decide to cash out to USD you will be given the equivalent amount of STEEM to equal 1 SBD. This way you know that when you exchange to USD you will be receiving the same amount. For example...hypothetically... let's say that 1 STEEM is valued at 50 cents. You will be given 2 STEEM to equal 1 SBD because 1 SBD is equal to $1.00. You can also use your SBD to promote your content on Steemit. By doing so you will expose your awesome content to more Steemians! Therefore giving your content the potential to receive more upvotes...thus creating more Steem in the form of Steem Power.

In regards to the second question about the "payment for 90 days steempower". Do I think it's okay? Let's take a look at the 90 days Steem Power delegation or just receiving upvotes organically. I can see a definite advantage of buying a delegation. By increasing your SP, it will further your content in the Steemit platform and reach more Steemians at a faster rate then doing it without a delegation (unless whatever you posted went viral). There is great potential in purchasing a 90 day delegation, but it also has the chance to go the other way. There are several variables to consider. For example is what your posting good-quality content, are you targeting the right audience with your tags, do you have a great title and possibly a photo to draw people in to read your post? (just to name a few) Overall...If I had to choose whether or not to buy a 90 day SP delegation. Personally I would take that risk. If it were financially within someone's means, I would recommend it to others to help give them a boost, but that's my own personal opinion. I'm not a financial advisor or crypto pro yet. This is just me speaking for myself. At the end of the day do your due diligence and work out the numbers before making the final decision. At least when you decide, you can say that you thoroughly researched it and knew your numbers. I hope this helps. Maybe a more experienced Steemian can give better insight to the pros and cons of a 90 day Steem Power delegation.

Best of luck and Keep on Steemin! :)