Bitcoin is a decentralized digital currency, without a central bank or single administrator, that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries. Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain.
Bitcoin is unique in that there are a finite number of them: 21 million.
Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies, products, and services.
 As of February 2015, over 100,000 merchants and vendors accepted bitcoin as payment..
Research produced by the University of Cambridge estimates that in 2017, there were 2.9 to 5.
8 million unique users using a cryptocurrency wallet, most of them using bitcoin..
The legal status of bitcoin varies substantially from country to country and is still undefined or changing in many of them. Whilst the majority of countries do not make the usage of bitcoin itself illegal, its status as money (or a commodity) varies, with differing regulatory implications. While some countries have explicitly allowed its use and trade, others have banned or restricted it. Likewise, various government agencies, departments, and courts have classified bitcoins differently.
China Central Bank banned the handling of bitcoins by financial institutions in China during an extremely fast adoption period in early 2014. In Russia, though cryptocurrencies are legal, it is illegal to actually purchase goods with any currency other than the Russian ruble..
Bitcoin has been criticized for its use in illegal transactions, its high electricity consumption, price volatility, thefts from exchanges, and the possibility that bitcoin is an economic bubble.
Bitcoin is still considered by many to be a risky investment and you should never invest more than you can afford to lose. That being said, investing in bitcoin has become much easier than it used to be, with several reputable companies now offering services to buy and store your bitcoins securely. Overall, it seems that bitcoin remains legal in the US; however individual states may have their own regulations concerning cryptocurrency exchanges.