Currently, a significant number of cameras support and natively capture images in DNG format. There is a very extensive list of cameras that are supported by the DNG format. You can find the list on the Adobe website here. RAW format is actually not a specific file format, it varies from manufacturer to manufacturer and from camera model to camera model. Some of the proprietary formats include: Canon’s .CRW and .CR2, Minolta’s .MRW, Olympus’ .ORF, and the various versions of Nikon’s .NEF.
For a concise explanation of RAW capture files, you may wish to read this article by Bruce Fraser called "Understanding Digital Raw Capture." Essentially, if you use your camera's built in converter to JPEGs you lose a whole lot of information captured by the camera. Using RAW files, you preserve the camera's data, albeit at a cost. The cost is the time and effort it takes to "develop" the RAW files into something you can use on the web or to send to a relative. Bruce Fraser gives the following analogy:
In some ways, it’s tempting to draw the analogy that shooting JPEG is like shooting transparency film while shooting raw is more like shooting negative film. With JPEG, as with transparency film, you need to get everything right in the camera, because there’s very little you can do to change it later. Shooting raw provides considerable latitude in determining the tonal rendition, like negatives, and also offers great freedom in interpreting the color balance and saturation. The fact that raw also lets you control detail rendition—noise reduction and sharpening—breaks the analogy but offers a further advantage.Now what does this mean to a real person doing genealogy? Using the DNG format gives you a possible way to preserve all of the information from your camera in a format that is likely to become one of the dominant methods of preserving that information. In using DNG you do not lose the original RAW data from the camera, all of that information is also preserved.
Now the downside. Most point and shoot cameras do not give you an option to save your files in any particular format. What you get when you download the image to your computer is what you get. You have no choice. Only higher end cameras provide the option of choosing the file format. As for scanners, they have some choices, but as of yet RAW is not one of them. Usually, with recent scanners however, you can save your files into the TIFF format.