Good piece with graphical descriptions and links.
In Open Data Science:
What is the Blockchain and Why is it So Important?
" ... Simply put, the Blockchain is a shared single version of the truth of anything digital. It is a database technology, a distributed ledger that maintains and ever growing list of data records, which are decentralised and impossible to tamper with. The data records, which can be a Bitcoin transaction or a smart contract or anything else for that matter, are combined in so-called blocks. In order to add these blocks to the distributed ledger, the data needs to be validated by 51% of all the computers within the network that have access to the Blockchain. .... "