The Morris Ring. Keepers of Morris lore and tradition for nearly 80 years. Good source of information on Morris dancing and related styles of dance.
The Morris Federation. Also an umbrella organisation for Morris sites with a reasonably informative web site. Charlbury Morris is a member of the Morris Federation.
The Morris dancing site. A portal to various Morris organizations, including those that are more relaxed about Morris tradition. Good sites for locating sides outside of the UK.
Icknield Way Morris Men. One of top Morris sides in the country. Their web site has excellent and accessible information on the various Morris dance traditions to help you understand what you are watching.
The village of Leafield is proud of its Morris history as the source of the Fieldtown Morris dancing tradition and more information about that can be found on the village website.
There are many sources of information on the possible origins of Morris dancing and its development over the centuries. For starters, the Wikipedia entry is a good overview, and there is a lot of information on the Morris Ring site. For a more personal perspective from a renowned student and teacher of the Morris (not to mention retired rocket scientist) Roy Dommett, you can find his lecture notes here. (You will need to explore the links to see the large amount of information available.)