One of the most significant evolutions in digital imaging was the introduction of RAW, the ability to record in-camera files of all the sensor information available at the time of exposure without additional processing. [Note that different authors use different spelling—RAW, Raw, raw—all referring to the same more or less “unprocessed” file.] This was envisioned as a means of recording capture data in a lossless fashion and protecting the original file from additional information being added. This way the photographer could return to the original “digital negative” at any time in the future with confidence it was unchanged and re-develop the image non-destructively as desired.
One niggle was that develop settings were recorded in an additional .xmp file that needed to accompany the corresponding RAW file; this occurred seamlessly as long as the image was being edited within Adobe programs, but could easily get separated when moving files by other means. Then along came DNG, Adobe’s version of a universal RAW file, to which addition data could be added (like the develop settings and other descriptive metadata). This was convenient since the pesky .xmp file was no longer required. However, DNG files cannot recognize all the proprietary and undocumented information of certain camera’s internal processors, and Adobe provides the DNG converter. See this article for more on the RAW/DNG debate
And then there is the question of RAW file compression is lossless or not. Sony has gained a big following with their extensive line of mirrorless interchangeable lens cameras, the latest being the a7R II. In this and many other models they use a two-part lossy compression routine while writing the RAW files to the memory card, kind of turning the original concept of “RAW” on its ear. Some photographers see this as anathema; others aren’t particularly bothered by it. If you use a Sony body or anticipate the purchase of one, you should be aware of this issue and understand it well enough to make an informed decision. This DP Review article describes the details of Sony’s RAW compression, and Michael Reichmann’s review of the a7R II includes his view that this isn’t as critical a matter in the real world as some believe. Sony has at least said they are aware of the concern and are looking into it. Time will tell.