Basically, blockchains can be classified into two types. The first is a Permissionless Blockchain, in which anyone can have access to the blockchain. The second is a Permissioned Blockchain, in which there is access only by known, trusted participants. There are also Private Blockchains and Public Blockchains. There can be multiple types of blockchains, and it is possible to have hybrids of these. Each type has its strengths and purposes. Each business, after careful consideration, should determine which is the most suitable option for their respective business.
Blockchains are comprised of a network of distributed nodes. Every node stores an exact and unchanging copy of the entire blockchain. The nodes validate and relay transactions to reach a consensus. The consensus is an important aspect of the blockchain. It is an agreement that the transaction actually happened. There is never a question of whether or not an agreement has occurred. The transaction is executed when consensus is reached.