Bitmap graphics, or just bitmaps, are image files that are built up using rows upon rows of coloured dots. They are also known as raster graphics.
These dots are called pixels. They are the bits in bitmap.
Common file formats for bitmaps are JPEG, GIF and PNG. These are all bitmaps but use alternative methods for compressing the data.
The alternative to bitmap graphics are vector graphics which use shapes rather than dots to define an image. Using bitmaps you can get a much more realistic image than a vector graphic, with all the shading and colour variations you would get in a photo. The downside however is that the image can’t be resized without losing quality. If you shrink it, it loses dots and loses that data forever. If you enlarge it, it just looks pixilated (each dot becomes a larger square).
When you take a photo using your digital camera, the data is stored in a long row of dots. These are the dots you then see on the screen as an overall image.