A cryptocurrency is a digital currency that has no physical expression in coins or banknotes. It is decentralized — there is no single center that stores information about transactions with it. 
Cryptocurrency is "created" by users — individuals or companies. It has a value that is formed based on supply and demand. You can buy and sell it, pay with it. 
Banks, the tax service and any government agencies cannot influence cryptocurrency transactions. All transaction data is stored in a blockchain registry or platform. 
Cryptocurrency can be used as ordinary money — for example, as a means of savings. But more often it is used as a means of payment and investment. 
Cryptocurrencies are considered a risky investment tool.  Most of the coins intended for investment are not backed by anything. Their cost can change rapidly for a variety of reasons.