To add to what Quartz said, pantos are always based on fairy tales, but incorporate lots of references to modern pop culture. They have a core love story (between the principal boy and a princess-type) accompanied by some sort of adventure.
Main characters:
The principal boy. Traditionally played by a girl but these days often played by a man. Usually wears tights and a jerkin or something similar.
The princess, or a female character similar to a princess. Should be pretty, usually a bit soppy.
The Dame. The main character of the play, really. She will be played by a man, and it’ll be a tall, overweight, manly man who’s not really trying to be like a woman at all. She will wear outrageous clothing, have most of the funny lines in the play and often have some sort of romantic interest with a baddie.
The Baddie. A really obvious baddie. When he appears on stage, to threatening music, the audiences boos and hisses. He gets the funny lines that wouldn’t work for the dame.
Buttons. A male friend/relation of either the principal boy or the princess; often in love with the princess. He gets some funny lines, but usually has some sad things happen to him that make the audience go awww.
A pantomime horse. Though it might be a cow or an elephant or a camel. Played by two people, traditionally, inside the horse, but these days often played by one person (maybe a child).
Lots of dancing townspeople.
All the characters talk to the audience now and then.
There are lots of songs and dances, including at least one song that the audience will join in on. Generally this is at the end of the panto, and the words will be shown on a huge board.
At some point there will be a competition where the audience is divided into two and has to sing or shout more loudly than the other side.
At some point, a character (usu. the Dame) will throw sweets into the audience.
And there HAS to be at least one occasion where a baddie will be behind a hero, and the audience will call out ‘he’s behind you!’ repeatedly while the hero and baddie turn round, with the hero not noticing the baddie until the audience has got really worked up.
This may be the same scene in which the call-and-response ‘oh no he isn’t,’ ‘oh yes he is!’ will come up.
I love panto, and get quite upset if I see one that’s veered too far from the mould.
I think I’m missing something here.