Bitcoin is the simplest way to exchange money at very low cost
Bitcoin is a zero trust consensus network that enables a new payment system and a completely digital money. It is the first decentralized peer-to-peer payment network that is powered by its users with no central authority or middlemen. It was introduced as open-source software by pseudonymous developer Satoshi Nakamoto that uses cryptography to control the creation and transfer of money.
As a start, think of Bitcoin as a currency, like the USD, AUD, HKD, CNY or EUR, only digital. They aren't printed or regulated by central banks like other currencies are. They are produced by millions of computers solving mathematical problems ensuring only a pre-determined number of coins enter circulation every day. These coins are rewarded to whoever is the first to solve the latest mathematical puzzle.
Bitcoin has many advantages. One of the biggest innovations is that it solves what is known as the double spending problem. According to the Wiki:
Double-spending is the result of successfully spending some money more than once. Bitcoin protects against double spending by verifying each transaction added to the block chain to ensure that the inputs for the transaction had not previously already been spent.
Other electronic systems prevent double-spending by having a master authoritative source that follows business rules for authorizing each transaction. Bitcoin uses a decentralized system, where a consensus among nodes following the same protocol is substituted for a central authority.