A bitmap (or raster graphic) is a digital image composed of a matrix of dots. When viewed at 100%, each dot corresponds to an individual pixel on a display. In a standard bitmap image, each dot can be assigned a different color. Together, these dots can be used to represent any type of rectangular picture.
There are several different bitmap file formats. The standard, uncompressed bitmap format is also known as the "BMP" format or the device independent bitmap (DIB) format. It includes a header, which defines the size of the image and the number of colors the image may contain, and a list of pixels with their corresponding colors. This simple, universal image format can be recognized on nearly all platforms, but is not very efficient, especially for large images.
Other bitmap image formats, such as JPEG, GIF, and PNG, incorporate compression algorithms to reduce file size. Each format uses a different type of compression, but they all represent an image as a grid of pixels. Compressed bitmaps are significantly smaller than uncompressed BMP files and can be downloaded more quickly. Therefore, most images you see on the web are compressed bitmaps.
If you zoom into a bitmap image, regardless of the file format, it will look blocky because each dot will take up more than one pixel. Therefore, bitmap images will appear blurry if they are enlarged. Vector graphics, on the other hand, are composed of paths instead of dots, and can be scaled without reducing the quality of the image.
Updated: February 6, 2014