Do you want the 1 minute answer? Or have you got a year or two to find out all about it?
I have to be careful - there are a lot of people who know a much better answer to this than I can give! But here's a start.
In short, "morris" is the generic name for several forms of English traditional display dancing. Typical forms are named after the areas of England where they are considered to have originated:
Individual dances for all the above are usually known by a combination of specific dance-name and place of "origin". The place name indicates the style of dancing in many subtle ways.
As well as the above, there are Longsword and Rapper dancing (done with different types of "sword"). Then there's Molly Dancing and..... well, you're into the beginning of the two year study now, I think!
So now you're going to ask: "Where can I find more information about morris dancing?"
There's a lot of information on the WWW. You might start with John Maher of Bristol Morris Men, who is also Overseas Bagman for the Morris Ring. His Web Page gives many useful links to other morris sides and information about folk dancing.
There is also an electronic discussion list for morris dancers worldwide, with daily exchange of news and views - particularly with the growing North American morris community. But there are also groups in Australia, the Netherlands and elsewhere.
You will also be able to find information on the three UK national morris organisations and the different roles they play.
Why not follow up some of the links and see where they take you?
But of course you will first want to read the rest of the St Albans Morris Men's Web Site won't you!
Go to S.A.M.M. Home Page.