Blockchain
is a global distributed ledger representing a network consensus of every
transaction that has ever occurred. Like the World Wide Web of information,
it’s the World Wide Ledger of value – a distributed ledger that anyone can
download and run on their personal computer.
Simply
put, think of the blockchain as a kind of a public spreadsheet. This
spreadsheet is stored simultaneously on a bunch of different computers and is
encrypted.
When
someone transfers a Bitcoin (or anything else you’re trading on the blockchain)
the transaction is verified by all of the computers, encrypted and added to the
spreadsheet, where everyone can see. The encryption and transparency are what
make the system secure.

Bitcoin
and other cryptocurrencies, such as Ripple XRP, sit on top of the blockchain.
They can be used as currencies, transferred between people just like normal
money. Or they can be used as a kind of token, the transfer recorded to signify
when something has been exchanged.
On the lighter side, here is what Scott Adams has to say: