What is different about a manatee from a dugong?

I was watching a vidoe about a suposed mermaid and it said it was most likely a dugong and they went and filmed a dugong and said then that if it was not it then it a manatee or seacow which I thought was excienct but I thought manatees and dugongs were the same animal but named different because people from different countries called them different things as for the sea cow are the even alive now?