Cryptocurrencies are digital or virtual currencies secured by cryptography, making them nearly impossible to counterfeit or double-spend. They exist on decentralized networks using blockchain technology, a distributed ledger enforced by a network of computers. Cryptocurrencies are generally not issued by any central authority, making them theoretically immune to government interference or manipulation. Advantages of cryptocurrencies include cheaper and faster money transfers and decentralized systems that do not collapse at a single point of failure. Disadvantages include price volatility, high energy consumption for mining activities, and use in criminal activities. 

Blockchain technology is central to the appeal and functionality of Bitcoin and other cryptocurrencies, serving multiple industries, supply chains, and processes like online voting and crowdfunding. Cryptocurrencies have various types, such as Ethereum’s ether, XRP Ledger Foundation’s XRP, and ETH, which were designed to facilitate work on the blockchain they are built on.

Cryptocurrencies are speculative digital assets secured by cryptography, making them a good investment. However, it is crucial to understand the risks involved before investing in cryptocurrencies, as they are relatively new technology.