RAW format is file type occurring at digital photocameras, scaners, etc. Basically it is raw format, which is not compressed and its print would be very expensive. It can be simply transferred to any other, simplier format, which can be for example PNG or JPG. It is not specified so there are many derivations, because every manufacturer of cameras is creating its own RAW format. Its main advantage is ability to take pictures with awesome quality. In fact there is nothing changed on the photography and that causes ability to do anything with unchanged photos and you can create almost magic with that. Then we have to mention zero compression ratio, but that causes enormous sizes of these files. For example picture in JPG can have 1 MB can in RAW format have 30 times bigger size – 30 MB.
To open files in this format you can use one of many programs. For example there is GIMP, which is available for free, but it needs some plugins for proper function. Then there is Adobe Photoshop, which can easily open these files which allows great modification options.