You want to try to get about 200 calories per hour, plus whatever your hydration needs are. Stay away from protein (eat that immediately following a ride). I find simple sugars to be easy to put down, which is why I mainly drink Gatorade. At full strength, it's 50 calories per 8 ounces, so if I drink a quart per hour, that's 200 calories/hour. However, often I don't drink the Gatorade full strength (maybe 3/4 strength, easy to do if you use the powder), nor do I always drink a quart per hour--that depends on temperature/humidity. I'll supplement with the PowerBar gel chew thingies (they're similar in texture to Gummie Bears), but fruit is also an excellent food during rides (bananas especially), if they're available.
Some people use the gel packs or Clif bars, which are also good.
At the end of the day, it really boils down to some degree of personal preference and some trial and error as to what your system can handle while you ride. On long tours, I like to stop in small towns and eat at the local hole-in-the-wall diner, but that also means I'm a little sluggish right after the meal for about a half hour or so.