The blockchain is a technology that can store and transmit information, it is secure and transparent. It doesn't need a control center to operate.
In practice the blockchain is a database in which has been recorded the history of exchanges between the various users who participated in its creation. This blockchain is thus shared by several users who can be anonymous or not, and who are direct actors in it. The string has a validity that can be verified through this history.
Some blockchains can be public and some others can be private. Private block chains will have limited access to a certain number of people. Only these can use them. The public blockchain can be summarized as a binder of historical data that can not be falsified and is anonymous. Everyone can read freely all the data contained in the blockchain; we can write inside it; and we can not destroy it nor erase it.
The first blockchain was created in 2008 with the bitcoin currency. This crypto currency was started by someone with the nickname of Satoshi Nakamoto. Nowadays, states and companies can use this technology to set up their own electronic currency in order to finance an economy or to get enough money to boost an activity.