Heartbleed is openssl vulnerability that targets servers. In this vulnerability, servers spits out more information than its required or asked and hence repeated request to server may cause leakage of sensitive information.The major concern in it is it is not detectable that is the server thinks it as a simple or common request.
So if you own a website or a security researcher then you may need to check whether the server is vulnerable to this attack or not. So check this type following command in nmap installed linux machine or in nmap running windows .
nmap -sV (server ip) --script=ssl-heartbleed
for example nmap -sV 192.168.0.104 --script=ssl-heartbleed
In next tutorial i will discuss about executing this attack to any server.