Also called an exposure sheet or x sheet. I prefer the term "dope sheet" because it kind of sounds dirty. This is a much simplified version of said sheet that I used for my "Obama in 30 Seconds" video (it's official title is "America Speaks"). All I was doing was lip sync, so I really only needed a way to chart the sounds for the mouth animation. I also added some visual cues for the eye elements. For film, this sheet would have a bold horizontal line every 8th frame (film plays at 24 frames per second, so each second gets split into three sections). I just animated this one for 30 frames per second, hence the bold line every 10th frame (though technically video plays at 29.97fps, but this piece wasn't long enough for that to throw me off). Also, this animation only contained one level, but that's not where you would place the action. Actually, now that I really look at it, I'm totally misusing this sheet, but since I was the only animator and the entirety of the art department, I think it's okay! The point was the lip sync.
I recorded the audio and imported it into Adobe Premiere at 30(ish) frames per second. That way, I had auditory and visual cues at every frame to tell me where the speaking sounds were. First, I listened for the hard consonants. They make a distinctive click at whatever frame they start. I'd write the consonant into the dope sheet. After that, I kind of filled in the rest of the sounds, indicating their length in frames on the sheet and drawing a line through the silence.
This particular animation was done in Flash only because I didn't have the proper setup to get everything aligned properly on paper (though all of the original drawings were done with pencil on paper and scanned into the computer). Because of this, my dope sheet became my best friend. It would appear that Flash has a habit of playing the video and the audio at different frame rates, so I just had to trust that as long as I had the "m" mouth shape where I had heard the "m" sound on the video that everything would sync up later! Luckily, it did. There's probably a way to fix this, but I don't have enough interest in Flash to figure it out.
In the meantime, I'm saving my money for a 2D animation program called Toon Boom. Flash wasn't meant to be a real animation program and Adobe hasn't seemed to catch on the the fact that people want to use it in this way (though the rumor is that they will at least be including inverse kinematics* tools in the next release of Flash). *the short answer to what this is is that it makes legs move like proper legs rather than blocks of wood on hinges.