If you had a PowerPoint presentation and delivered it to your students.
Some would be able to use it as they had a computer with Microsoft Office installed.
Some may be able to view it as they downloaded and installed the PowerPoint viewer or OpenOffice
Some would be able to use it as they had a PDA which could convert PowerPoint into a portable format.
Some versions of Microsoft Office for example do not include PowerPoint.
However if you saved the presentation as a series of JPGs then users who had no access would then be able to view the slides on a variety of devices, including their mobile phones, iPods, PSPs, PVRs, even digital cameras as well as PDAs and PCs. Even some TV devices have memory slots allowing you to play digital images on the television.