What does a full node do?

Their main tasks include maintaining the consensus between other nodes and verification of transactions. They also store a copy of the blockchain, thus being more secure and enable custom functions such as instant send and private transactions. When making decisions for the future of a network, full nodes are the ones that vote on proposals.

What is a full node & a lightweight node?

Full nodes which store a copy blockchain and thus guarantee the security and correctness of the data on the blockchain by validating data. The second type is a lightweight node – each user participating, who needs to connect to a full node in order to synchronize to the current state of the network and be able to participate.

What is the difference between a full node and an archive node?

The main difference between a full node and an archive node is that the archive node keeps track of all of the chain's previous states (i.e. a complete picture of the Ethereum ecosystem), while a full node only needs to store a snapshot of current state data.