File formats such as tiff, jpeg, bmp and gif are different than word processing or database files in that they are raster image file formats consisting of pixels.
Pixel means picture element so in other words, they are a picture file.
Therefore, when we discuss the resolution of a file, we are referring to the number of pixels in a raster image file. All raster images are made up of pixels laid out in a grid. So when an image is 300 dpi that means there are 300 pixels for every inch of image. To get a better idea of how pixels make up resolution, visualize a checkerboard. It is a grid of 8 squares tall by 8 squares wide for a total of 64 squares or pixels.