A node in crypto is nothing more than a computer or a similar device that is part of the blockchain network. These nodes keep track of transactions, check their authenticity, and also help prevent problems like double-spending (using the same cryptocurrency twice).
The blockchain network is not monitored by a central authority. The nodes keep track of everything. For instance, when someone sends a Bitcoin, nodes all over the world check the blockchain history to make sure the sender has enough money. They send the transaction to other nodes if it is valid, and it is finally added to a block in the blockchain network.
There are three types of nodes:
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Full nodes: they have the heaviest responsibility. They download and store the entire blockchain—currently over 700 GB for Bitcoin—allowing them to fully and independently validate every transaction and block.
Light nodes: people also call them SPV nodes. They don’t download the full blockchain. Instead, they just pull in the basic outlines (the block headers) and whatever details they need. They let the full nodes handle most of the checking. That’s what makes them perfect for phones or anything that can’t handle a giant download.
Mining nodes: they always run alongside a full node. They round up all the transactions waiting to get confirmed, put them into a proposed new block, and then battle it out with other miners to solve a crypto puzzle. The one who cracks it first gets to slap that block onto the chain and walks away with the prize—freshly created coins and the fees from those transactions.
This approach is precisely why cryptocurrency does not require middlemen like banks. The nodes carry out all verification tasks. There are simple requirements to be a part of the node system: a computer, software, and an internet connection.