The default optimization setting, 'Screen', is suitable for most purposes. Files will look great on screen, but may not be suitable for high quality printing.
'Print' can be used if high quality printouts are required. Files will be print-quality, but file sizes will be slightly larger.
'Mobile device' can be used if files only need to be viewed on cellphones or PDAs. File sizes will be tiny, but picture quality may be poor on larger screens.
If none of the above options meet your requirements, you can select your own, custom optimization settings.