A node is a part of a blockchain that communicates with other nodes.
Any computer that contributes to maintaining a Proof-of-Work blockchain is called a node. Nodes are not the same as miners, in fact, nodes do not do the difficult calculations that miners do. Nodes merely store blocks. This requires less powerful hardware, so normal computers can also be a node. On the other hand, nodes do not receive rewards like miners. Nodes are therefore mainly run by enthusiasts and hobbyists.
A miner must always run a full node in order to select valid transactions to form a new block. If it does not have a full node it cannot determine which lying transactions are valid or not. If a node cannot verify that the balance (on the blockchain) matches the transaction then it cannot execute a transaction.
At the same time, a miner is always a full node. A node is not necessarily a miner; a node is merely a receiver and sender of blockchain data. A full node has a full copy of the blockchain transaction history.
Proof-of-Stake blockchains also use nodes. An important difference is that with PoS, there are thus no miners. A number of PoS blockchains also contain so-called masternodes. A master node is generally heavier than a regular node. It always does other things besides validating and sending transactions. For example, if a vote takes place, it is usually kept up to date by a master node. Masternodes exist only with PoS blockchains.
The main difference is in the size of the memory and the power which is many times higher with a master node. A master node is generally always online. In most cases there is a reward attached to this, but this also depends on which cryptocurrency the masternode is running.
