A full node is a network connected computer that runs the official cryptocurrency client and has a complete and synchronized copy of the blockchain.
When Bitcoin first appeared, everyone running the client program it was a full node. Not only that but every client was a miner too and could earn a few BTC per day just to keep the software running.
We all know how mining changed with GPU’s and ASIC’s and suddenly mining using a regular CPU-run program was no longer profitable.
So the early concept of a full node which would also mine coins was soon changed to the current meaning: a node which has a complete sync’ed copy of the blockchain in it and which can broadcast and receive transactions from other P2P network users, thus helping to propagate data in the network.
If it isn’t a full node, then what is it?
Light wallets are the alternative to full nodes.
With lightweight wallets the blockchain is not downloaded to the client and the client trusts a centralized service to process its transactions.
Electrum Wallet is one such solution for Bitcoin and MyEtherWallet is a similar application for Ethereum.
Both these systems run on local web interfaces and have a programming library that can generate, sign and the send transactions to a central server. This central server must be trusted, of course, for this system to run.
Lightweight wallets break cryptocurrency decentralization by requiring a central trusted authority.
Running a Full Node
The only way to be fully decentralized is to have a copy of the blockchain on local storage. Currently, any major cryptocurrency will require hundreds of gigabytes of hard disk space and they’ll grow tens of megabytes per day.
Running a full node requires permanent and stable network access in order for the node to keep up with the network and to be able to participate in it. Bitcoin network usage is approximately 5 Gigabytes per day. Therefore it is not suited for limited connections.
Running a full node is a matter of downloading the official client for your cryptocurrency of choice and keeping it running. Care must be taken not to download a fake or malware version of the software. Make sure to only access the official channels for your cryptocurrency. If in doubt, search for your cryptocurrency on Crypto.BI or go to CoinMarketCap.com and look for your cryptocurrency there. The description page will have a link to the official website. Some scammers are able to manipulate search results to place their malware high up. Do not trust search results, always look for an official reference link.
Running a full node is a matter of maintaining ample disk space and stable internet connection. As long as the client is running, it does the rest on its own.
Illustration Credit: Syp via Wikimedia Commons by CC