Node

Basics

• Sometimes also called Validators or Clients.
• From this post: 

"Node: A node is a point of intersection/connection within a network. In an environment where all devices are accessible through the network, these devices are all considered nodes."

• A computer or specialised hardware that will look at a transaction as it arrives and then run a series of checks to verify it. This checking is also called validating. Therefore nodes are often also called validators.
• Each node builds its own transaction pool, which are mostly the same.
• Within Bitcoin the conditions can change and evolve over time and a present list can be checked through the AcceptToMemoryPool, CheckTransaction & CheckInputs functions in the bitcoin client.

Node Types

• The most common node types are Full Nodes, clients, Light Clients, Masternodes and Validator Nodes.

Full Node

• Any computer that connects to, for example, the Bitcoin network is called a node. Nodes that fully enforce all of the rules of Bitcoin are called full nodes.
• Full nodes download every block and transaction and check them against the specific blockchain's core consensus rules.

Client

• Some nodes are clients. A, for example, bitcoin client is the end-user software that facilitates private key generation and security, payment sending on behalf of a private key, and optionally provides:

1. Useful information about the state of the network and transactions.
2. Information related to the private keys under its management.
3. Syndication of network events to other peer clients.

Light Client

• Aka light node, light wallet or Simplified Payment Verification (SPV) clients.
• Most nodes on the network are lightweight nodes instead of full nodes, but full nodes form the backbone of the network.

• SPV is a method for verifying particular transactions were included in a block without downloading the entire block. The method is used by some lightweight clients, this is also why they call wallets that use this "Light Wallets". It helps with having wallets on mobile phones for example.

Validator Node

• Validators are nodes but often with more PoS functionalities.
• From the Polkadot FAQ (6-5-2021):

"Other projects sometimes have a different definition of validator that approximates more closely to remote signing keys without the full operation of a validating node. On Polkadot, each validator is running their own validating node and performing full verification of the Relay Chain, voting on finality, producing blocks in their decided slots, and verifying parachain state transitions. Other projects may consider validators and "validating nodes" as separate entities."

Masternode

• A Masternode is a crypto full node (computer wallet) that supports the network by hosting an entire copy of the coin's ledger in real time. They also provide other services, which ones depends on the blockchain. In return, the Masternode will receive crypto coins as a reward. It is an alternative to mining.