Let's start with the big one. Plug the question into Google and you'll get any number of attempts at an answer. Blockchain is like the net of a soccer goal, like
DNA, like our
"pattern of thought", like
Google Docs (this one's actually pretty good).
But it's not nearly as complicated as these extended metaphors would have you believe. Seriously.
The blockchain is a method for digital record-keeping that doesn't require a central authority to verify its data. That's it.
Obviously there's a lot more complexity when you get into the actual execution of making those digital records secure, but that's it. It's also a deceptively simple idea, because the potential applications for the blockchain are seemingly infinite.
Imagine your bank without any wire fees or ATM charges; a digital music file that can't be pirated from a musician; or Facebook without Mark Zuckerberg. Theoretically all of these things are possible (as well as some admittedly preposterous ideas) because of blockchain technology.