Bitmap graphics consist of a grid, or raster , of tiny squares called pixels . Vector images are made up of mathematically defined lines and curves called vectors. Graphics must be in bitmap format for Web use. A pixel is also the smallest visible colored dot the monitor can display. The higher the resolution, the more pixels in the image and the larger the image file size.
Bit refers to an electronic signal that can be on or off.
Bit depth is a measurement of the number of bits of stored information per pixel. This determines how much color information is available for each pixel in the file. Greater bit depths yield more colors, more accurate color representation and larger file sizes. A pixel with a bit depth of 8, referred to as 8-bit color, has 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 or 256 possible colors. A pixel with a bit depth of 24 has 16 million possible colors.