A Cryptocurrency (or crypto currency) is a digital asset designed not to require the backing of a physical good like gold and silver, but to be self-sustaining and self-regulating. A cryptocurrency is a part of blockchain technology, a decentralized ledger of all cryptocurrency transactions that have ever been executed. To pay someone in cryptocurrencies you need to access their public key and sign your transaction with it.
A cryptocurrency is a form of digital asset or currency that uses cryptography, the process of converting legible information into an almost uncrackable code, to track purchases and transfers. Protected by strong cryptography, there is no central authority for a cryptocurrency like Bitcoin and no single point of failure. Most cryptocurrencies are decentralized and exist on distributed ledgers.
When the novelty of Bitcoin wears off for the average person, the value proposition is going to be too difficult to understand in order to get them to participate. That is when a market for cryptocurrencies will emerge, which will be driven mostly by investment services and applications built on top of it. The concept of a cryptocurrency has existed since the 1980s, but actual implementation wasn't really feasible until computers and networks became more powerful. Most cryptocurrencies today use blockchain technology similar to what Bitcoin uses (although they all have different ledgers), there are also many projects trying to improve on the original idea.
Cryptocurrencies are used to pay for goods and services and exchange for other currencies. They use cryptography to secure their systems and verify transactions. You can hold cryptocurrency units as an investment or receive them for online purchases.