Heartbleed is a much-publicized security flaw in the OpenSSL cryptography library; an update to this OpenSSL flaw was published on April 7th, 2014, which was (coincidentally?) the same day that the flaw was disclosed.
OpenSSL runs on secure web servers certified by trusted authorities; it is estimated that about 17% of secure web servers may be vulnerable to an attack based on the Heartbleed flaw, which could compromise the server’s private keys and end-user passwords and cookies.
Fortunately, most organizations with secure web servers have taken steps to identify and fix this flaw. And, to date, no known exploitations of this flaw have taken place.
Unfortunately, this flaw has been around for over two years and leaves no traces; if exploited, there would be no ready evidence that anything was wrong.
At the moment, there is not much any end-user can do except to logout of any secure web server that has not been patched. (See http://filippo.io/Heartbleed/, a site created by Italian cryptographer Filippo Valsorda, which claims that it can identify unpatched servers.)
Http://money.cnn.com/2014/04/09/technology/security/heartbleed-bug/index.html contains an informative article and video by Jose Pagliery at CNN Money.