How do cymbals work in Rock Band?

I'm wondering how cymbals work in Rock Band. The drums that ship with the game only have four drum pads, and I know that you can buy a set of three cymbals, but do the songs actually distinguish between drums and cymbals, or is hitting (for example) the blue cymbal the same as hitting the blue drum pad?

If they are the same, is there any point to having the cymbals?

The old RB2 cymbals are just for show - they don't add anything to the game. Hitting the green cymbal, for instance, is the same as hitting the green pad.

However, the new RB3 cymbals serve a real purpose - they allow you to play in the new "Pro Mode," in which you are required to differentiate between hitting the cymbals and the toms!

Here you can see the pro-mode drums being played with cymbals (middle track). The green and blue ovals coming up are cues to hit the green and blue cymbals, respectively. The usual blue rectangles coming down the screen are the cue to hit the blue tom instead.

Note that this image also shows pro-keyboard (left) and pro-guitar (right) as well. (Credit to IGN for the image)

For Rock Band 1/2, there is no indicator on the screen that says "this is a cymbal hit".

In RB2, during fills, they produce a cymbal sound when hit (as opposed to a tom hit).

As for the point, Rock Band 3 will introduce a drum pro mode which will require you to hit the proper cymbal / tom as opposed to just hitting the right colour. Other than that, it's all for good fun!