Stainless Steel - I think this is the most common type of material used on watches nowadays for its durability and ease of production and costs. Steel watches can come in two types of finish which is either polished chrome like finish for those who like the shiny "bling" and a brushed or matte finish for those who like the more suttle industrial "tough" look. Steel watches are usually heavy so the nigger the watch, the heavier the heft on your wrists. On a plus side, polished finish steel watches can be re-polished if some scratches appear after a long period of use.
A polished finish(see how shiny it is) and a brushed finish
PVD Coated Stainless Steel - This type of watches are one that give the "Gun Metal" look. These watches look very nice and rugged and usually in dark colours like black, dark gery, or dark brown. Its basically a stainless steel watch which is coated with the coloured material. Although they might look good eventually the coating will give way and if you scratch your PVD watch... there is no way of polishing it up again.
A brand new PVD watch and a used PVD watch
Gold - Oooooh the expensive gold. At one point of time gold watches were all the rage but not anymore as gold watches now just seem to look cheesey (but thats my personal opinion). Expensive to make, high cost and difficulty in maintanence probrably brought its downfall. Anyways any gold watch user would never have peace of mind wearing it as it attracts many would be robbers. Just as in jewelry it cannot be made of 100% gold anyways because if it did it would be easily dented.
A famous gold watch brand back in the 80's
Resin - Usually in black, Casio watches have a lot of models that use resin's for their watch cases. Tough and less prone to scratches it has proven to be a popular choice for sport watches. The toughest resin made watches are of course the Casio G-shocks. Resin watches are tough but it also depens on the humidity of the country that you live in. Where I come from (Malaysia) the humidity is high so resin and rubber watches do not last as long. (althouth its still quite long). Resin watches usually have a stainless steel caseback.
A mud resistant resin watch
Plastic - Plastic cased watches were made popular by Swatch watches and was usually design in frech funky designs and one ca also see the watch mechnism. The advantage is of course the watches were relatively in-expensive. Plactics watches are also really light weight, suitable for those who need to tell time but dont like the feeling of anything heavy around their wrists. Swatch watches also use plexi-glass on the dial making it easy to polish off any scratches on the face.
Plastic cased watch
Titanium - Ahhh the metal used on spacecraft and highspeed aeroplanes. This space age material has the durability of steel without the weight. Get this type of watch material if you like steel watches but do not like the heft of heavy watches. The lightweight often makes users forget that they are wearing a watch. One downside to titanium is that it scratches mush easier than stainless steel and only comes in matte finish. Best to use a nato or rubber strap with titanium watches if you have a rugged lifestyle.
Titanium. Although it looks big..its light!!
And there you have it, the many types of watch materials to help you choose the best watch for you. Anything I missed? Lets not mention exotic materials like white gold and platinum as its quite rare on a watch.