In layman term, cryptocurrencies or crypto are a decentralized digital form of currency. However, this definition does not even begin to describe what it really is and the technology behind it. To put in in a more technical way, cryptocurrencies are limited entries in a database that can only be changed when a set of rules are fulfilled. The technology behind this digital currency system is called blockchain, a term which is becoming exponentially widespread over the last few years.
It is basically a public ledger where all the transactions ever done within the network is available to everyone in that network. And whenever someone in the network buys or sells a unit of the cryptocurrency, an entry is made in that ledger that cannot be changed, making the transaction official.
The most common forms of cryptocurrency nowadays are bitcoin, litecoin, and ethereum. However, every month new ones are launched with even tech giants like Facebook planning to release its own cryptocurrency.