It’s easy to understand the Heartbleed bug
One of the most important bugs discovered this year is the Heartbleed bug. The bug affects the OpenSSL cryptography library, which is a widely used implementation of the Transport Layer Security (TLS) protocol.
In jargon, this kind of vulnerability is classified as a buffer over-read, which means that a software allows more data to be read than should be allowed.
Surfing in the Web I found a nice picture which explains in a very understandable way what are the effects of this bug.
#####How the Heartbleed bug works, credits http://xkcd.com/1354