Heartbleed is a critical bug in the OpenSSL’s implementation of the TLS/DTLS heartbeat extension that allows attackers to read portions of the affected server’s memory, potentially revealing users data, that the server did not intend to reveal. Two-thirds of the world’s servers i.e.. half a million servers are affected by the HeartBLEed Bug, including websites, email, and instant messaging services. The bug is a vulnerability resided in. certain versions of the popular open source encryption standard OpenSSL, a popular version of the Transport Layer Security (TLS) protocol.
Source: https://thehackernews.com/2014/04/heartbleed-bug-explained-10-most.html