The Heartbleed bug allows anyone on the Internet to read the memory of the systems protected by the vulnerable versions of the OpenSSL software. This allows attackers to eavesdrop on communications, steal data directly from the services and users and to impersonate services and users.
Affected users should upgrade to OpenSSL 1.0.1g. Users unable to immediately upgrade can alternatively recompile OpenSSL with -DOPENSSL_NO_HEARTBEATS.
Read more here - http://heartbleed.com/