An imaged reduced in file size for web display is described as ‘optimised’.
A JPEG is a file format used to reduce the file size of an image. It is best used to compress photographic or continuous tone imagery. (For images that use flat-colours and where maintaining a consistent hue is important, GIF or PNG file formats should be used.)
The JPEG format uses an algorithm to average the colour values across an image. Averaging the colour values reduces the number of instructions required by the computer to recreate the image and the fewer instructions, the smaller the file size.
When the JPEG algorithm is applied, the tonal range of the image is reduced (or ‘compressed’). In a black-and-white photograph, the 100% white highlights and 100% black shadows are clipped. The new, compressed colour values vary, depending on amount of compression applied.
In graphics programs, how much an image is compressed is referred to in terms of ‘quality’, either as percentage scale (e.g. 100%-0%) or descriptions such as ‘best’, ‘better’, ‘good’, etc. The higher the quality the larger the file size and the better the image reproduction.
100% quality/‘best’ quality = no compression = larger file size
0% quality/‘good’ quality = maximum compression = smaller file size