What is the difference between Nessus and Metasploit?

Put simply, Nessus identifies potential vulnerabilities, whereas Metasploit exploits the vulnerability. Metasploit is basically a list of exploits you can use against a target, Nessus tells you which one you should pick a bit like asking a shop assistant what would work best to hack the said computer, the shop assistant (Nessus), will say: "Yes of course sir, I think you will want this." as he points to an item on the shelf (Metasploit).