I am sure that there are tons of descriptive videos on youtube. But in a nutshell they are both massive communication applications. One uses email as the main protocol, the other uses chat protocol.
Your experience will vary depending on the email client or IRC client that you use.
Operating systems might have different clients built only for that operating system like Outlook, or Claws for email or Konversation for Linux or mIRC for windows.
The best way to use it has to do with time and also the rest of the community, an empty chatroom would be hard to get any benefit from it. So is a quiet ML. But ML tend to go on for days, while chat will come and go in minutes so speed is a big difference. Also not everyone is connected and aware of what its been chatted. So there is another issue about loosing conversations while ML everyone gets an email so they could reply 2 days after the conversation was initiated.