More recently, Bitcoin has become a method for making ransom payments — for example, when your computer is taken over by so-called ransomware.

Why won’t the government just shut it down?

The records of the Bitcoin network, including all balances and transactions, are stored on every computer helping to maintain the network — about 9,500 computers in late 2017.

If the government made it illegal for Americans to participate in this network, the computers and people keeping the records in other countries would still be able to continue. The decentralized nature of Bitcoin is also one of the qualities that have made it popular with people who are suspicious of government authorities.

Can Bitcoin users give themselves more Bitcoins?

Anyone helping to maintain the database of all Bitcoin transactions — the blockchain — could change his or her own copy of the records to add more money. But if someone did that, the other computers maintaining the records would see the discrepancy, and the changes would be ignored.

Are there legal uses?

Only a small percentage of all transactions on the Bitcoin network are explicitly illegal. Most transactions are people buying and selling Bitcoins on exchanges, speculating on future prices. A whole world of high-frequency traders has sprung up around Bitcoin.

People in countries with high inflation, like Argentina and Venezuela, have bought Bitcoin with their local currency to avoid losing their savings to inflation.

One of the most popular business plans is to use Bitcoin to move money over international borders. Large international money transfers can take weeks when they go through banks, while millions of dollars of Bitcoin can be moved in minutes. So far, though, these practical applications of Bitcoin have been slow to take off.