What causes rust?

Okay, maybe a lame question, but what causes rust? I think it has to do with salt because the ocean seems to rust stuff. Am I close?

I was taught in school that if a certain chemical in metal comes into contact w/ water/rain for a while, it will rust

I learnt this quite recentyl in chemistry..I don't think you're quite there..
It's just the metal reacting (iron and its other alloys eg steel) reacting with the oxygen in the water (rain). This forms iron oxide which is called rust. The product (iron oxide) is the orangey red substance that we can see.

When certain metals touch certain molecules in the air, water, or rain, they combine and make iron oxide most commonly known as rust.

If you really want to know funmail me and I'll give you a big explanation lol.
(its too big to write in here lol)

x0x0

The answers on here so far are pretty much correct - it is basically metal, air and water reacting together to create Iron Oxide (rust). If you only have air at metal, it will not rust; similarly if you only have metal and (un-aerated) water, it will not rust either. Salt does not have to be present to create rust, but if it is, the creation of rust is much more severe because of the salt's reaction with the air, metal and water. Salt water is also much more abrasive than fresh water also. The purer the fresh water is, the less severe the rust is.

For instance, in my country our railways dumped a few steam locomotives in places for protection from waterways in the 1920's-1930's. Up in the mountains where the water is very clean and the air is also very fresh, the rust on these locos dumped in the 1920's is little more than a liberal coating of surface rust - you can quite literally sandblast and paint the bits and they're as good as new. However, locos dumped where salt water is present in the 1930's are little more than twisted crumbing wrecks of metal - no amount of work will ever make them good again, they are just too far gone. The Titanic is in far better condition than these latter engines, the only reason it doesn't look like the day it sunk is the presence of air particles in the salt water and the great pressure on the ship at that depth. But a ship sunk in fresh water will show very little rust. The difference salt makes is dramatic, but by no means a necessary agent in the creation of rust.

oxidation causes rust!!!