It’s rather obvious that Lego bricks are made from plastic, but there are many kinds of plastic.
Is the recipe for LEGO bricks plastic secret?
Actually, it depends on the part.
Most parts are indeed ABS as it's a very sturdy plastic, but sometimes it's not suitable, or downright impossible to use. I've seen a presentation on quality issues given by a LEGO employee in LEGOworld a few years back, and as far as I remember there were between 10 and 20 sorts of plastic in use. The ones I remember are:
And the list goes on. I know the presentation hadn't been made public, but some people took pictures, so if I ever find it back I'll add a link.
EDIT: still can't find it, but I know it was given at LEGOworld in 2006 by Bjarke Schønwandt who was Global Quality Manager at LEGO at the time (he may still be, I have no idea).
According to a letter from a LEGO representative posted at this site:
Tires and elastic materials found in some LEGO sets are manufactured from Styrene-Butadiene Styrene (SBS), a material commonly found in vehicle mud guards and shoe soles.