Blockchain is a cryptographically secured, time-stamped, public and distributed database of every bitcoin transaction that has ever occurred on the network. “Distributed” here means that the information in the blockchain is broadcast to and recorded by every node in the network. There is no one central database. Any user can refer to this list of transactions and check exactly what how many bitcoins have ever belonged to any specific address at any point in time. This way the system is transparent, double-spending is prevented, and there is no need for a trusted central authority.
The bitcoin blockchain is supported by the efforts of many miners: users who put computing power to the task of solving a mathematical problem that will reward them with bitcoins, while at the same time producing the hashes that secure the blocks of data in the chain. The hashes are cryptographically produced strings of data that make the info in the blockchain virtually impossible to tamper with.