The blockchain is a combination of logic and technical instruments known previously but combined together by “Satushi Nakamoto” in a way that is totally new. An invention that made things considered to be impossible in IT become possible.
A blockchain is a chain of blocks (series of documents), of which each has a predefined structure, each contains a ledger of current entries (for example transactions), each is related to the previous block, each is created within a specified period of time. In the Bitcoin blockchain blocks have few additional attributes; Each block is created in different versions by different creators, each final block is chosen to be the one through voting (Consensus), block creator can be everyone and everyone can contribute to entries registered within the block. To create a block creators have to contribute a huge effort in computing power and energy (Proof of Work) and for this they are receiving payments.
The blockchain invention solved one of the main logical and technical problems in IT that is called the double spending problem.
The double spending problem is basically the possibility of sending a digital document to several recipients simultaneously with all the copies having the same value as the original. Actually all copies are originals. In some fields this is not a problem as in sharing photos for instance or text messages. The problem becomes more obvious when it relates to sending money without the intervention of third parties such as banks.