What is Bitmap?
Most images you see on your computer are composed of bitmaps. A bitmap (also known as raster images) is a map of dots/pixels, or bits (hence the name), that looks like a picture as long you are sitting a reasonable distance away from the screen. These tiny dots of individual color come together to create the image you see on your screen. Common bitmap filetypes include BMP (the raw bitmap format), JPEG, GIF, PICT, PCX, and TIFF. When you’re working in Photoshop you’ll usually be working on a bitmap image. Because bitmap images are made up of a bunch of dots, if you zoom in on a bitmap, it appears to be very blocky. Vector graphics (created in programs such as Freehand, Illustrator, or CorelDraw) can scale larger without getting blocky.
Take a look at this close up of an eye. You don’t notice the dots because they are usually very small.
I’ve found a beautiful example of how these dots working together make an image. This fun yet informative movie shows how to make a photograph using a paint ball gun on a motorized robot!