As the general definition goes, cryptocurrency is a digital or virtual currency protected by cryptography, which makes it nearly impossible to counterfeit or double-spend.
Many of them are available on decentralized networks based on blockchain technology, a widely distributed ledger, the latter meaning a database that is synchronously shared in consensus across multiple computers.
The first blockchain-based cryptocurrency was Bitcoin, which is still the most notorious.
Today, there are thousands of other cryptocurrencies, each with their own functions and specifics. Some are Bitcoin clones or forks, while others were built from scratch, the biggest example being Ethereum.
But what does "decentralized currency" mean?
Traditional currency is centralized, meaning that it is managed and controlled by central organizations, such as banks and governments, while cryptocurrencies can be issued by non-central authorities.
People are essentially their own banks, as transactions follow a peer-to-peer model with no third party acting in the middle. They are responsible for their own funds, for as long as they have them stored on their web3 wallet (like Metamask or Trustwallet), though they may also choose to store them in centralized exchanges (like Binance or Coinbase).