Used for logos and illustrations as a vector file. Can be scaled to any size without loss of quality, supports transparency for large format printing.
Used as a way to share documents without losing the design or quality. Generally used for emailing and sharing as well as printing.
Used for high quality photography and low res online use. Will lose quality if scaled over its pixel width.
Designed for transferring images on the internet, not made for print. Supports transparency and online use.