From Wikipedia: en.wikipedia.org
Bitcoin (sign: BTC) is a decentralized  digital currency   based on an open-source,  peer-to-peer Internet protocol. It was introduced in 2009 by a pseudonymous developer (or developers) Satoshi Nakamoto.  Since 2011 Bitcoin has been criticized as not being a functional currencydue to its experimental nature, volatile markets,  and allegedly limited use in trade.  
Bitcoins can be transferred through a computer or smartphone locally or internationally without an intermediate financial institution.  Each bitcoin is subdivided into 100 million smaller units called satoshis, defined by eight decimal places. 
Bitcoin has no central organization and is not managed by any government or central authority. Instead, it relies on an Internet-based peer-to-peernetwork. The creation of new bitcoins is automated and given to servers or "bitcoin miners" that confirm bitcoin transactions as they add them to a decentralized and archived transaction log approximately every 10 minutes.
The transaction log is authenticated by end-users through hashed ECDSA digital signatures (similar to a username and password) and added to through calculations of varying difficulty, performed by dedicated servers called bitcoin miners. Each 10-minute portion or "block" of the transaction log has an assigned amount of bitcoins that is awarded to miners (who can then sell them at their discretion) once it is added to the log and confirmed by other miners. The amount of bitcoins per addition depends on how long the network has been running and how much in transaction fees has been paid. Currently, 25 new bitcoins are generated with every 10-minute block. This will be halved to 12.5 BTC during the year 2017 and halved continuously every 4 years after until a hard limit of 21 million bitcoins is reached during the year 2140.  
Bitcoin is the most widely used alternative currency   and accepted by various merchants and services internationally.