What a lot of people can probably tell you is that a lot-a-lot of years ago, somebody called Guy Fawkes tried to blow up the Parliament in London. He got caught, and on Guy Fawkes' Night, people celebrate by lighting bonfires, having fireworks and making "a guy" to burn ceremoniously (think a scarecrow).
Children learn the rhyme:
Remember, remember the fifth of November
Gunpowder, treason and plot.
I see no reason, why gunpowder treason
Should ever be forgot.
As is usual, over time, certain details get forgotten, and others get amended. For full details of Guy Fawkes' Day, have a look at good old Wikipedia: http://en.wikipedia.org/wiki/Guy_Fawkes_Night
For a children's version (or for grown-ups who don't want to plough through lots of information), have a look here: http://www.bbc.co.uk/newsround/15351828
Sheffield (where we live) has a fireworks display every year on the 5th of November, called "After Dark" - you can see the 2011 one here: http://www.youtube.com/watch?v=fcbnP09TGk0 - Toby hasn't been yet, because it starts quite late...maybe next year.
Bonfire night is also the time of warming autumn recipes, treacle, toffee, and a special type of gingerbread from our region, called parkin. There are lots of recipes for bonfire night here: http://www.bbc.co.uk/food/occasions/bonfire_night - we'll definitely try a parkin one this year!