A cryptocurrency is a medium of exchange like normal currencies such as USD, but designed for the purpose of exchanging digital information through a process made possible by certain principles of cryptography. Cryptography is used to secure the transactions and to control the creation of new coins. Unlike centralized banking, like the Federal Reserve System, where governments control the value of a currency like USD through the process of printing fiat money, government has no control over cryptocurrencies as they are fully decentralized.
All confirmed transactions from the start of a cryptocurrency’s creation are stored in a public ledger. The identities of the coin owners are encrypted, and the system uses other cryptographic techniques to ensure the legitimacy of record keeping. The ledger ensures that corresponding “digital wallets” can calculate an accurate spendable balance. Also, new transactions can be checked to ensure that each transaction uses only coins currently owned by the spender.
The anonymous nature of cryptocurrency transactions makes them well-suited for a host of nefarious activities, such as money laundering and tax evasion. The first cryptocurrency to capture the public imagination was Bitcoin, which was launched in 2009 by an individual or group known under the pseudonym Satoshi Nakamoto. As of September 2015, there were over 14.6 million bitcoins in circulation with a total market value of $3.4 billion. Bitcoin's success has spawned a number of competing cryptocurrencies, such as Litecoin, Namecoin and PPCoin.
Cryptocurrencies are backed by math rather than the word of a government or financial institution. While they, like all currencies, still depend on their perceived value, their scarcity is based on math and cannot be adjusted by any one group or person. They are neither tied to the availability of physical goods, such as gold, nor can they be artificially created by governments or financial institutions like dollars can. Cryptocurrencies use a distributed network to allow for a p2p (peer-to-peer) transaction system without the need for third parties. In order to keep this secure, cryptocurrencies utilize mathematical algorithms and a public ledger.
National currencies are protected by banks and a variety of government controls that generally work to control inflation, prevent malicious practices, stamp out counterfeiting, adjust related interest rates, and many other important currency management decisions. Cryptocurrency doesn’t have this kind of support (not yet, anyway). It often depends entirely on miners and the encryption process for protection and control. This naturally comes with its own risks, and those risks can make people less willing to invest.