Bitcoin is a digital currency. It is described as a "Bitcoin: A Peer-to-Peer Electronic Cash System" in the Bitcoin Whitepaper.
Bitcoin Core is a software application that implements Bitcoin.
Anyone can implement a software application to interact with the Bitcoin network (network of computers that all agree on the same protocol rules). So perhaps it is most accurately described as a network that follows a strict protocol similar to email. There are many email clients, but they can all communicate with each other.
Bitcoin is an experimental digital currency that enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate with no central authority: managing transactions and issuing money are carried out collectively by the network. Bitcoin Core is the name of open source software which enables the use of this currency.