A node is essentially a copy of a blockchain stored on an electronic device that’s connected to the internet.
Nodes are responsible for maintaining a copy of a blockchain, and in some cases processing transactions on said blockchain.
As blockchains exist across a distributed network, nodes are an integral part of keeping them operational.
Almost any electronic device, such as a computer, phone, or even a printer, can act as a node, as long as it’s connected to the internet (and therefore has an IP address). A collection of these nodes is known as a “network”.
Owners of nodes are willingly contributing their device’s processing power to store and validate the blockchain’s transactions – known as mining – in return for rewards in the form of the blockchain’s underlying cryptocurrency.
Nodes come in two forms: “full” and “light”. A full node contains the entire list of every transaction that occurred on a blockchain. A light (or “lightweight”) node will only contain a segment of a blockchain’s history, using a trusted full node as a reference.
Full nodes help to maintain a blockchain, and are also used to validate transactions, meaning miners will most often use them. They also help to enforce the blockchain’s security and stability; the more there are, the more secure it is.
Light nodes on the other hand don’t require a full copy of a blockchain, and therefore tend to be favoured by those using smaller devices (such as mobile phones).
Still not sure you understand? Take a look at the Bitcoin wiki:
“Any computer that connects to the Bitcoin network is called a node. Nodes that fully verify all of the rules of Bitcoin are called full nodes.”