Bitcoin is a decentralised currency that was defined in a concept paper written by the mysterious Satoshi Nakamoto, who remains unknown to this day, in 2008. The idea came from the concept that money should not be controlled by one authority that we are expected to trust. It should be controlled by the people who use it and the trust would be created through a secure computer network that would verify every transaction.
Today, Bitcoin is both a currency, as it was originally envisioned but is also an investment option like property or shares.