What is Bitcoin? This is the most basic question this website seeks to answer and yet the one we will most likely find most difficulty with. Technically speakling Bitcoin is a digital curreency in which an immutable record of transactions is curated and new units of currency are generated or "mined" by the computational solution of mathematical problems.
Bitcoin is the original cryptocurrency built on blockchain technology. It is described in the Bitcoin Whitepaper as having been designed as a peer-to-peer electronic cash however some argue it has become much more then that and some would argue it is much less or something even worse. Bitcoin has no central bank or administrator. Bitcoins can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries.
Bitcoin is probably best understand in simplistic terms as a digital currency run by the bitcoin infrastructure of specialized computers all over the world that power bitcoin mining or the mathematical computations take place in the discovery of every bitcoin block.
For now we'll point you to the original source of bitcoin discussion and the Bitcoin Whitepaper as posted by Satoshi Nakamoto in 2009. It's not honest to attempt to understand bitcoin without first reading the source.