What is a light node?
Light Node downloads all transactions and histories to ensure that all participants follow the crypto-currency consensus rules. It enforces all the rules of a consensus. If one of the participants violates the rules, he or she can easily be rejected by this node.
The following scenario could be an example of this system: a block generating more than a certain number of bitcoins could be rejected by these in the network.
Some full nodes also serve as light node (also known as lightweight clients). Lights verify transactions using a method known as simplified payment verification (SPV).
SPV uses a node to check whether a transaction has been included in one block, without having to download the entire blockchain. SPV means that full nodes can serve light nodes by allowing them to connect and transfer their transactions to the network.
The relationship between light node and full node
A light node only has to download the headers of all blocks on the blockchain. This means that download and memory requirements are much less intense than with a full node.
The relationship between full nodes and light ones is important because otherwise light ones could not connect to the cryptocurrency network. It is also important to note that SPV nodes effectively rely on full nodes to ensure that the blocks and transactions are properly validated according to consensus rules.