Crypto-currency is a medium of exchange which is created and stored electronically in the form of a block-chain, using encryption techniques for controlling generation of monetary units and verifying transactions.

In simple words, cryptocurrency is like an online database with limited entries that cannot be modified without meeting explicit requirements. The database contains details of accounts, balances, and transactions. Currency can only be moved between different accounts for a transaction when appropriate conditions are fulfilled and a verification entry is then added to the database.