These are the three JPG formats for image compression, which affects the way your images display on the web:
Baseline Standard is used when you want your JPG to be recognizable to most web browsers. It’s basic and, well…. standard! It makes the least amount of changes to your image. All web browsers support it.
Baseline Optimized optimizes the color quality of the image and produces a slightly smaller file size (2 to 8% – a little more compression, or slightly faster loading). All modern web browsers support it, but not some of the earliest versions of web browsers.
Both Standard and Optimized produce good quality images, so it’s really just a matter of choice… or habit. And I guess my choice is Optimized.
Baseline Progressive creates and image that will display gradually as it’s downloaded… remember the old days of painfully slow browsers when photos loaded on a page in a series of lines instead of all at once? We’d never use that one anymore.
via Selecting Baseline ‘Standard’ vs Baseline Optimized.