Raw files aren't really images as such, but the data straight off of the camera's sensor. I think of it as freezing the state of the camera immediately after the exposure has finished, and bundling it up into a file, normally with some kind of lossless compression. The wikipedia article provides a fair amount of detail about the sort of thing that's recorded, but as a guide I'd say:
- All the levels readings from the sensor
- Some information about the sensor type (so the Raw converter knows how to interpret the sensor data)
- All of the settings currently applied to the camera
- The time and other EXIF-type meta-data associated with the image
- Possibly a JPEG thumbnail image
Bear in mind that there is no single Raw format, and all camera manufacturers are free to bundle up this sort of data in their own way. Adobe is trying to promote its DNG (Digital Negative) format, which I personally use, as a standard RAW format. A few cameras can output DNG, but most have to be converted. I do this on import into Lightroom, as one of the advantages is that it can store Lr edit data within the file, and not require xml sidecars. Another advantage of DNG is that it tends to result in smaller file sizes than native Raw files.
I would imagine that because of how most sensors are built with a Bayer filter-type design, the data from the sensor just contains levels from each photo-site on the sensor. A bitmap, for example, would have combined the RGB sites to form a single pixel.