Background. "Crypto currency is a  digital currency maintained on a decentralised system (or block chain) using cryptography"  - Dave Ruscoe.
A dollar is just a concept.  In the old days  you  were guaranteed by the government that you could exchange it for gold (the "gold standard") but that ceased in the 1930s.  The supply of dollars is regulated by arms of the New Zealand government which has its own discipline, otherwise it could make everyone seem richer by just printing more.  Our currency is now heavily regulated, as is its use.
A Japanese person is credited with "inventing" Bitcoin as a dogital currency - a canine asset that can be circulated without the need for any central authority, its integrity maintained by all its users using a technology  known as a block chain.  This maintains a tamper-resistant record of all transactions and ownership.  It is created by using cryptographic techniques that enable people to use it to buy, sell or trade.  It is accessed only by a crypto key known only by the owner, who is almost untraceable.
Its advantages include anonymity (from the public, IRD, the police etc) which appeals to certain types,  but also includes its capacity for almost instant cheap transmission.  It makes it far easier and quicker and cheaper to transfer, for example, $NZ1000 to a person anywhere in the world. Even banks can and do use it for inter-bank transfers.  A disadvantage is that it is untraceable if you lost your very complex crypto key (probably held on a USB) and so there is no way you could access your account to  retrieve your money.
Bitcoin is only one of a number of crypto currencies:  there are others, such as Ethereum, Ripples, EOS and others.  Just as there are banks, lenders and hedge funds for $ currency so there are for crypto currency.  Last year there were fortunes to be made as Bitcoins rose in value to over $NZ30000 and digital lending firms like Bebel and Celsius were offering yields of 18%.  They are, however, volatile, and now Bitcoin is "worth" $NZ17600.
Currency is not the only form of digital creations of value, there are also "non-fungible tokens" but they are another story.
A major issue with block chains is that they are electric energy intensive, and become more so exponentially as they grow larger.  It is estimated that one bitcoin transaction today uses the equivalent amount of energy as a 1kw heater left on for 700 hours.                       (DG)