That is a very broad question. Do you mean as an illustrator file, a preprint file, a photograph, etc.? Each file format offers something unique and beneficial. For example:
AI: Adobe Illustrator AI files are vector files used by designers and commercial printers to generate files of various formats and sizes.
EPS – EPS encapsulated Postscript files are often used by designers to transfer an image or artwork, typically a vector file, to another application.
PDF – Portable Document Format A PDF is a universal file format that preserves/embeds the fonts, images, layout, and graphics of any source document, regardless of the application used to create it.
I can go on with PSD, JPG, GIF, TIF, PNG… What are you specifically trying to achieve?