A cryptocurrency (or crypto) is a cryptographic (= encrypted) electronic currency - supported by a decentralized computer network - whose transactions and transmission are based on cryptographic algorithms. 
The number of units in circulation and the maximum money supply are, in principle, defined in advance and visible to all. Until proven otherwise, a cryptocurrency cannot be counterfeited or usurped; furthermore
It does not depend on a central bank or a government

The principle of value scale that defines a cryptocurrency is the same as that of the current currencies (or FIAT currencies, i.e. €, $) that we use daily; however, unlike other currencies, it is only virtual. 
Therefore, there is no banknote or physical cryptocurrency coin. 

The other difference is that this currency is encrypted, i.e. it can only be used by a person who has the code to decrypt the currency. 
Cryptocurrencies operate using a computer technology called Blockchain (or secure public registry). And it is precisely these networks, these blockchains, that make cryptocurrencies the most secure currencies in the world

A cryptocurrency is not necessarily intended to provide a payment system between users; it can allow for remuneration, an exchange of services, loans, etc.
Everything that is done with traditional currency is, therefore, reproducible and possible in the same way with a cryptocurrency.

The first to have been created, and the most emblematic of all, is, of course, the Bitcoin.
Alongside the leading cryptos (Bitcoin, Tether, Ethereum, Monero, EOS, Ripple, Litecoin, etc.), thousands of other cryptos (or AlterCoins) are trying to make their mark. 
Each cryptocurrency is listed on one, or often multiple, non-harmonized marketplaces or platforms. Hence, there are many possibilities for arbitrage.