Bitmap is a general description of a raster image made up from pixels as opposed to mathematically defined vectors. Hence, GIF, JPG, PNG, PSD and TIF are all bitmap images, though the manner in which the formats store and display pixels may vary.
When a vector image is opened within a photo editing application like Adobe Photoshop, the user is presented with a dialog box in which dimensions and pixel resolution must be specified. This process is called rasterizing the image.