How Heartbleed works thanks to XKCD posted April 2014
I found a pretty nice explanation of Heartbleed for the layman in this XKCD comic. Heartbleed is a recent and alarming vulnerability found in the OpenSSL toolkit that serves most of the application/websites today. To quote Schneier:
"Catastrophic" is the right word. On the scale of 1 to 10, this is an 11.
Here's the comic:
And if you want to dig a bit more into it, you can read some more explanations on security.stackexchange.