Node is a computer that connects to Bitcoin network and uses p2p protocol that allows nodes communicating with each other within the network as well as spreading information on transactions and blocks. Information is distributed among such nodes and they are what block chain network consists of.
Nodes that fully enforce all the rules of bitcoins are called full nodes. This means that they are fully synchronized with block chain network, i.e., they store 100 Gb of a download set of block chain files on a hard disc drive to date. Most nodes of the network are lightweight nodes but full nodes form its backbone.
Full nodes operation
Full nodes download every block and transaction and check them against Bitcoin's core consensus rules. For example, blocks may only create a certain number of bitcoins, transactions should have correct data format and signatures of bitcoins spent, a transaction output cannot be double-spent within a single block chain, etc.
If a transaction or block violates the consensus rules, then it is absolutely rejected by a node, even if all other nodes of the network think that it is valid. Moreover, for some time it will cease to “notice” the source of transaction that has been trying to download it while violating the consensus rules. It is one of the most important characteristics of full nodes: they do what`s right no matter what, thus, ensuring high level of transactions security.
What makes a full node different from a lightweight node?
Lightweight nodes are not as independent as full nodes and they do what they are told to by the majority of miners. This is why, they can be temporarily tricked so they would accept a transaction or block that is not actually valid. This can cause serious financial damage, especially for those web-sites that automatically process bitcoin transactions.
Running a full node is the only way you can use Bitcoin in the safest possible way. They store all the information about the network due to what they ensure that all the rules are being followed – for example, that all the bitcoins are spent by the owner and that no bitcoins were spent twice, etc.
Why should the owners create full nodes?
It should be noted that ownership of a full node and its participation in transaction validation is not mining (the search for new blocks in the network for a reward), i.e., your presence in a block chain network itself won`t guarantee you any profit. Running a full node is the safest possible way to download transactions with the use of cryptocurrencies. Besides, stability of the whole system operation depends on full nodes stability of operation.