What is a node, and how does a peer-to-peer network of nodes store the ledger?

A node is a single computer running the blockchain software and holding a full copy of the ledger; the network is thousands of such independent peers, each an equal, all keeping the same history.

There is no central server. Instead, many independent participants each run a node — one instance of the blockchain software with its own complete copy of every transaction ever made. These peers are all weighted equally and constantly compare their copies with each other. Because the data is replicated across everyone, the ledger is extraordinarily durable: if your computer and its backup both die, your data survives on all the other nodes, and the system keeps running as long as even one honest node remains. Anyone can join by meeting modest hardware requirements and running the software, which is precisely what keeps the network open and decentralized — the barrier to participation is deliberately low.

Go deeper:

• Peer-to-peer — Wikipedia — the networking model in which every node is an equal peer, with no central server.