Imagine that a company owns a server farm comprised of 10,000 computers use to maintain a database holding all of its client’s account information. This company owns a warehouse building that contains all of these computers under one roof and has full control of each of these computers and all the information contained within them. This, however, provides a single point of failure. What happens if the electricity at that location goes out? Or its internet connection is severed? What if it burns to the ground? What if a bad actor erases everything with a single keystroke? In any case, the data is lost or corrupted.
What a blockchain does, is to allow the data held in that database to be spread out among several network nodes at various locations. This not only creates redundancy, but also maintains the fidelity of the data stored therein: if somebody tries to alter a record at one instance of the database, the other nodes would not be altered and so would prevent a bad actor from doing so. If one user tampers with Bitcoin’s record of transactions, all other nodes would cross-reference each other and easily pinpoint the node with the incorrect information. This system helps to establish an exact and transparent order of events. This way, no one node within the network can alter information held within it.
Because of this, the information and history of transactions of a cryptocurrency) is irreversible. Such a record could be a list of transactions (such as with a cryptocurrency), but it also is possible for a blockchain to hold a variety of other information like legal contracts, state identifications, or a company’s product inventory.