STEEM can turned be into STEEM Power (SP) by powering up in your steemit wallet or cli_wallet. It can also be turned into Steem Dollars (SBD) by using the market' option in the steemit wallet (click on your STEEM amount)
STEEM and Steem Dollars can be transfered to other user accounts. Which enables you to send funds to friends, donate to causes or pay for goods.
They can also be sent to cryptocurrency exchanges such as Bittrex or Poloniex (Bittrex.com; Poloniex.com) and then exchanged into Bitcoin (BTC). Once exchanged into Bitcoin it can be exchanged for other cryptocurrencies or fiat currencies such as USD, GBP, EUR, CNY, and RUB.