Bitcoin is a decentralized peer-to-peer cryptocurrency that is based on work published by Satoshi Nakamoto. The first Bitcoin transactions happened on January 3 2009.
Today Bitcoin Core, as the original Bitcoin source code is called, is maintained by a tightly knit group of developers referred to as the "core team". With input from the community, these folks maintain and evolve the Bitcoin surce code. Bitcoin is unique in the fact that all its business rules are contained within the source code. There is no external document dictating what Bitcoin really is - Bitcoin is whatever is programmed into Bitcoin Core. The evolution of this source code follows a workflow denominated BIP's. These stand for Bitcoin Improvement Process proposals and can be submitted by anyone. For example, the request to increase block sizes is a BIP. The request for separate transaction and signature data (segregated witness) is also from a BIP. These BIP's get agreed upon or are discarded by the community, in a democratic system that has worked well for the past 9 years.
Bitcoin uses cryptographic technology developed over many decades, including proof-of-work, irreversible hashing algorithms, the Byzantine General's problems, and many other academic research ideas which were incorporated into the code we've come to know as simply Bitcoin.
Today, the token representing value in a Bitcoin transaction, which is what we usually call Bitcoin, is worth over U$ 8000 and by some estimates it could be worth as much as tens of thousands of dollars within a very short period. The demand for these coins has sparked an entire industry made up of miners, speculators, investors, entrepreneurs and inventors from all walks of life.
Scientists work to develop new protocols for Bitcoin, developers work to create new and innovative products on top of Bitcoin, speculators generate massive liquidity in cryptocurrency exchanges and the whole ecosystem works thanks to this self organized community that formed spontaneously around Bitcoin.