Bitcoin is a global interconnected computer network made up of tens of thousands of nodes.
A computer network comprises two or more computers that are connected—either by cables (wired) or WiFi (wireless)—with the purpose of transmitting, exchanging, or sharing data and resources.IBM
A node is a connection point inside a network that can receive, send, create, or store data. A node is essentially [a computer] that can recognize, process, and transmit information to any other network node.IBM
The nodes [computers] that make up the bitcoin network all run the same free open source software. This software is a language that allows these computers to interact with each other, and it governs the rules of the system.
Specifically, bitcoin is a peer-to-peer network, where every computer can communicate with any other computer directly. There’s no central server processing all of the data, and every computer is the same in the eyes of the network. There are no hierarchies, there are no master nodes that have any more power than any other nodes. Peer-to-peer means that bitcoin has a flat topology, every node is equal.
The nodes constantly pass around verify transactions, and importantly, all of these computers have the same copy of the ledger. When someone makes a bitcoin transaction all computers in the network update their ledger together. Therefore bitcoin is on all of the computers in the network at the same time.
Because all nodes run the same software, all members of the network must adhere to the consensus rules and monetary policy in order to send and receive transactions. This makes it impossible for one single person or group to cheat the system or adjust the rules like they can with fiat money.
You can think of the bitcoin network kind of like a big mushroom, where there’s not really a brain. It’s all just one big blob. Or you could even think of Bitcoin as the brain itself, with electricity pulsing throughout the network to build one self-sufficient, autonomous, system.
And to confuse you even more, bitcoin is also the name of the software that the nodes are running on the network.
So to recap: bitcoin the currency, is sent over Bitcoin the network, by computers running Bitcoin the software.