It's a descendant of the folk-plays once widespread in Britain. Within communities, groups of men (sometimes boys) would join together to celebrate some important date in the year by acting out some play appropriate to the time, in the streets or inns, or even private houses. They were usually a way to gather contributions, in cash or kind, to the welfare of "the labouring classes".
They usually marked a change of season. The Christmastide mummers are sometimes claimed to be a relic of the Winter solstice festivities, celebrating the turn of the year when the days once again start growing longer, ie the "death" and "resurrection" of the Sun. In January, there were Plough Plays, then Springtime (Easter), and Autumn (the Souling plays of All Souls Day, November 1st).
No-one knows for sure, but the form we know seems to be derived from a small number of chapbooks which appeared about 250 years ago. Of course, the chapbooks were perhaps just the first printed collations of plays which were in the oral tradition. What is sure is that the "chapbook" plays returned to the oral tradition; they were handed down, with variations, from generation to generation of Mummers.
Are they related to the Mystery Plays?
No, they're purely secular, they were not "sponsored" by the Church. Also, the language and action is broader and (we hope) more comic. There's a touch of Shakespeare's "Rude Mechanicals" from about the mummers.
Yes, many of the plays are described as "Hero/Combat", i.e. a Hero (St George) fights and conquers all manner of enemies (The Dragon, The Turk, Bold Slasher,etc). The other major motif is "death and resurrection", so inevitably all of the Hero's victims get "cured", and live to fight another day.
Its just an old word for Actors or Players.
Why the blacked-up faces?
The convention used to be that the character in the play was NOT the person who played him, so the mummer's identity was hidden by make-up or a mask or obscuring costume. Naturally, everyone knew who was who, but even a player's mother used to address him as his Character and not by his name!
Are St Alban's the only Mummers around?
By no means. Use your favourite WWW search engines to search for "Mummers", and you may find other groups; with luck, there will be one near you.
I'm totally enthused by this! Where do I get comprehensive, in-depth information?
The Traditional Drama Research Group runs the Folk Play Research Home Page at http://www.folkplay.info/. This should tell you everything you want to know; if it doesn't, it contains links to many other sites that cover the subject in great depth.