How it works
Transactions are written into blocks, and each block contains a fingerprint of the previous one. Changing an old transaction would mean recomputing every later block faster than the rest of the network. Miners verify blocks and are rewarded with new bitcoins.
Pizza for 10,000 BTC
On 22 May 2010 programmer Laszlo Hanyecz bought two pizzas for 10,000 bitcoins. The first known purchase of a real item with bitcoin is now celebrated as Bitcoin Pizza Day. At later prices those pizzas were worth hundreds of millions of dollars.
Risks
The price of bitcoin swings wildly. Cryptocurrencies are also used in scams and ransomware. Beware of “investment opportunities” from ads and social networks; they are often fraud. This is not investment advice.