Full node is an implementation of a blockchain protocol. It maintains a complete copy of the entire blockchain ledger and participates in validating transactions and blocks as part of the consensus process. A full node also stores all transaction data, smart contracts and other information related to the network’s operations. Full nodes are actively involved in the verification process, helping to ensure that the network remains secure and reliable for users. 
As the most important part of any decentralized network, full nodes play an essential role in maintaining trust and security within a blockchain ecosystem.  With full nodes, users can trust that the network is operating accurately and securely. They also help to ensure faster transaction speeds and better scalability for blockchain networks. Without full nodes, a network would be prone to various attack vectors and other malicious activity. Thus, full nodes are essential for keeping blockchain networks safe, secure and functioning properly.  Ultimately, full nodes are important components of a blockchain system that provide users with trust and security in the network.  They are essential for the integrity and health of any blockchain-based solution.

Bitcoin Core: Bitcoin Core is the original software implementation of the Bitcoin network, and it is a full node client that downloads and stores the entire Bitcoin blockchain. It also allows users to send and receive Bitcoin transactions, mine Bitcoin, and participate in the network's consensus process.
Ethereum Geth: Geth is the official Go implementation of the Ethereum protocol and is a full node client that allows users to interact with the Ethereum network, deploy smart contracts, and participate in consensus. Geth can be used to sync with the entire Ethereum blockchain or to run a local development node for testing purposes.
Litecoin Core: Litecoin Core is the official full node client for the Litecoin network, which is a cryptocurrency that is very similar to Bitcoin. It downloads and stores the entire Litecoin blockchain, allowing users to send and receive Litecoin transactions, mine Litecoin, and participate in consensus.

A blockchain full node is like a library where all the books are kept. Each book is like a block of information and the library has all the books in a series in order. The books in the library are like the information on the blockchain, and the library is like the full node. Just like how a library allows people to come and read books, a full node allows people to see and access all the information on the blockchain.