Blockchain is a distributed list of public or private ledger recorded in the blocks. It is a foundational technology of most of the cryptocurrencies on the Internet. Most famous among all is Bitcoin.

You can virtually imagine it as a long list of memory blocks connected to each other sequentially in a very very knit manner. Where these blocks are stored redundantly in multiple computers spread all over the world making it highly available & fast. Everything in those memory blocks is stored as a transaction.  Those transactions are confirmed by the participating computers through some consensus algorithms making it highly difficult to forge, delete or modify the transactions. Though transactions are visible to every connected computer but yet no one can trace that who are the parties associated with those transactions. Until unless those parties declare it by themselves.

Data stored in Blockchain is highly secure & immutable. In short, it means it is very very difficult as well as expensive to forge or modify a transaction and it is extremely easy to detect the inconsistency & forged data/transaction/stored information.