What is Blockchain?
A blockchain is basically a public ledger of all bitcoin transactions that have ever occurred. A block is what records the current transactions and once the transaction is completed, it is transferred to the blockchain as permanent database. Each time a block is completed, a new one is created. Blocks are linked to each other (like a chain) in proper linear, chronological order with every block containing a hash of the previous block.
Take conventional banking as an analogy, the blockchain is like a full history of banking transactions. Blocks can be seen as individual bank statements, and bitcoin transactions (what’s recorded in the block) are entered in blockchain as bank transactions would be recorded.
Blockchains contains every Bitcoin transaction that has been ever executed. It thus allows us to learn about facts like how much value belonged to a particular address at any point in the past.
Here is a video that would help you get familiar with Blockchain in two minutes: