From its name, it can be inferred that cryptocurrency is a cryptographically secured currency.
Decentralization and digitalization are some of the identifying features of this currency. Bitcoin is considered to be the premier decentralized crypto as others before it didn't demonstrate any success. Along with the formerly mentioned conditions, an asset is sustained through consensus to classify as a cryptocurrency.
Blockchain, a public ledger, records, and stores the currency's transactional data across distributed nodes and contribute to its decentralization.
Its characteristic anonymity has given crypto a bad reputation as it can be favorable for conducting illicit activities.
Bitcoin alternatives, called altcoins, are Litecoin, XRP, etc.