Blockchain is a decentralized and distributed database in the open source model in a peer-to-peer network (without central computers and without a centralized data storage space). Blockchain is used to record individual transactions, payments or accounting entries coded with cryptographic algorithms. In other words, it's a public, anonymous and verifiable register that anyone can access.
For better understanding, split there two words - block and chain. A block is digital information stored in a public database called a "chain". Blockchain is a growing list of records linked through cryptography. Each block contains the previous block cryptographic hash, time stamp and transaction data. Blockchain records transactions between the parties in a permanent and secure manner. It is open (accessible to everyone), and at the same time impossible to overwrite and edit - thus it is resistant to manipulation.