Bitcoin is the first and most famous digital currency, which operates on a decentralized basis. Bitcoin transactions and issuance are done by a peer-to-peer network of computers; there is no central authority. Bitcoin was created in 2008 by a person or a group of people going by the pseudonym of Satoshi Nakamoto.
Bitcoin transactions are verified through the process of mining: miners get new Bitcoins as a reward for verifying transactions and creating new blocks. The new blocks are recorded in a public distributed ledger called blockchain.
Bitcoin is the blockchain with the biggest hashing power in the world.
More recently, a communication protocol layer was built on top of the Bitcoin blockchain, called Omni Layer (formerly known as Mastercoin). Omni layer allows the creation of custom tokens and cryptocurrencies on the Bitcoin Blockchain.