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.
Satoshi Nakamoto is the name used by the unknown person or people who designed bitcoin and created its original reference implementation. As part of the implementation, they also devised the first blockchain database.
In the process they were able to solve the double-spending problem for digital currency. They were active in the development of bitcoin up until December 2010.
Nakamoto is estimated to have mined one million bitcoins before disappearing in 2010, when he handed the network alert key and control of the code repository over to Gavin Andresen. Andresen later became lead developer at the Bitcoin Foundation.
 Andresen then sought to decentralize control. This left opportunity for controversy to develop over the future development path of bitcoin..