Steem, Steempower and Steem Backed Dollars SBD

There are three types of cryptocurrency on the Dteem blockchain. Two liquid and one iliquid. This is there description from the Steem Whitepaper.

Steem​ ​(STEEM)

Steem is the fundamental unit of account on the Steem blockchain. All other tokens derive their value from the value of STEEM. STEEM is a liquid currency, and therefore can be bought or sold on exchanges,​ ​as​ ​well​ ​as​ ​transferred​ ​to​ ​other​ ​users​ ​as​ ​a​ ​form​ ​of​ ​payment.

Steem​ ​Power​ ​(SP)

Start up companies require long-term capital commitment. Those who invest their money in a startup expect to wait years before they can sell their shares and realize their profits. Without long-term commitment, a startup seeking to raise additional capital through the sale of additional shares would be competing with existing shareholders looking to exit. Savvy investors want their capital contributions to grow​ ​the​ ​company,​ ​but​ ​growth​ ​cannot​ ​happen​ ​if​ ​the​ ​new​ ​capital​ ​is​ ​given​ ​away​ ​to​ ​those​ ​looking​ ​to​ ​exit.

There is significant value to having long-term commitment because it enables communities to make long-term plans. Long term commitment of stakeholders also causes them to vote for long-term growth rather​ ​than​ ​short-term​ ​pumps.
In the cryptocurrency space, speculators jump from cryptocurrency to cryptocurrency based mostly on which one is expected to have short-term growth. Steem wants to build a community that is mostly owned and​ ​entirely​ ​controlled​ ​by​ ​those​ ​with​ ​a​ ​long-term​ ​perspective.

Users are able to commit their STEEM to a thirteen week vesting schedule, providing them with additional benefits within the platform. STEEM that has been committed to a thirteen week vesting schedule is called Steem Power (SP). SP balances are non-transferrable and non-divisible except via the automatically recurring conversion requests. This means that SP cannot be easily traded on cryptocurrency​ ​exchanges.
When users vote on content, their influence over the distribution of the rewards pool is directly proportional to the amount of SP that they have. Users with more SP have more influence on the distribution of rewards. This means that SP is an access token that grants its holders exclusive powers within​ ​the​ ​Steem​ ​platform.
SP holders are also paid interest on the balance of SP that remains vested. 15% of the yearly inflation is paid to SP holders as interest. The amount of the interest that they receive is directly proportional to the amount​ ​of​ ​SP​ ​they​ ​hold​ ​relative​ ​to​ ​the​ ​total​ ​amount​ ​of​ ​vested​ ​SP​ ​across​ ​all​ ​users.

Transferring from STEEM to SP is referred to as “powering up”, while transferring from SP to STEEM is
referred to as “powering down.” SP that is powered down is returned to the user over a period of weeks,​ ​via​ ​13​ ​equal​ ​weekly​ ​payments,​ ​starting​ ​one​ ​week​ ​after​ ​the​ ​power​ ​down​ ​is​ ​initiated.

Addendum: the recent HardFork changed the power down time to 4 weeks.

Steem​ ​Dollars​ ​(SBD)

Stability is an important feature of successful global economies. Without stability, individuals across the world could not have low cognitive costs while engaging in commerce and savings. Because stability is an important feature of successful economies, Steem Dollars were designed as an attempt to bring stability​ ​to​ ​the​ ​world​ ​of​ ​cryptocurrency​ ​and​ ​to​ ​the​ ​individuals​ ​who​ ​use​ ​the​ ​Steem​ ​network.

Steem Dollars are created by a mechanism similar to convertible notes, which are often used to fund startups. In the startup world, convertible notes are short-term debt instruments that can be converted to ownership at a rate determined in the future, typically during a future funding round. A blockchain based token can be viewed as ownership in the community whereas a convertible note can be viewed as a debt denominated in any other commodity or currency. The terms of the convertible note allow the holder to convert to the backing token with a minimum notice at the fair market price of the token. Creating token-convertible-dollars enables blockchains to grow their network effect while maximizing the return for​ ​token​ ​holders.
Steem Dollars are referred to with the symbol SBD, an acronym for Steem Blockchain Dollars. Creating SBD requires a combination of a reliable price feed, and rules to prevent abuse. Providing a reliable price feed involves three factors: minimizing the impact of an incorrect feed, maximizing the cost of producing an​ ​incorrect​ ​feed,​ ​and​ ​minimizing​ ​the​ ​importance​ ​of​ ​timing.

These currencies are related to each other via two smart contracts.

@lalai