Picture files come in two fundamental varieties: Raster and object-oriented.
Bitmap pictures (sometimes called raster pictures) are made up of individual pixels (tiny dots). The pixels align on a grid, which your eye blends into a single image.
Color mode describes the way colors are represented in a file; bit depth is the number of bits used to represent each pixel. The simplest color mode is 1-bit (also known as “line art” or “black-and-white”). More complex images, such as photographs, have depth because they contain multiple-bit pixels that can describe many levels of gray or color.
Dimensions describe the physical size of a picture (for example, 3″ x 5″). The dimensions of a picture file are determined by the application that creates it, and dimensions are stored in the picture file.
Resolution is the number of pixels (dots) per inch in a picture. Resolution is dependent on dimension. In other words, if you change a picture’s dimensions, you change its resolution too. For example, consider a 72 dpi picture that’s 1″ x 1″. If you scale that picture to 200% after importing it, its effective resolution drops to 36 dpi, because the pixels are enlarged. To determine the effective resolution of an imported picture, use the Effective Resolution field in the Classic tab of the Measurements palette.
Object-oriented pictures contain information that describes how to draw the position and attributes of geometric objects. You can then reduce, enlarge, stretch, and rotate these pictures without worrying about how they will look — object-oriented pictures look smooth, no matter what their scaled size may be, because the are not made up of pixels.