Camera Raw Files are used to store data coming directly from the image sensor of a camera without processing. It is the image data as seen by the sensor before any modifications by software have taken place. Photos taken from the camera application on your smartphone, or even most digital cameras, are "pre-processed" with internal software unless you specifically change the settings in your device to store raw images (this functionality is not supported by all devices). Thus, raw image files are typically much larger in file size than pre-processed images.
Sometimes also known as digital negatives, a raw image file is normally much larger in size and saved in a raw file format. The process of converting a raw image into a viewable format is also known as "developing the raw image". There are many raw file formats in use today and vary across manufacturers and devices.