From Robin Good at 
"Screen sharing is generally understood as the technologically-empowered ability to transmit the contents of your computer screen to one or more remotely connected Internet users.
In other words screen sharing allows you to show PowerPoint presentations, Word documents, images and any other software running on your computer while remotely connected users see in real-time what you see on your screen.
Traditionally screen sharing has been long referred to as application sharing, to define more specifically the ability not just to show specific applications to other connected participants but the combined ability of screen sharing with remote control.
So while the terms "application sharing" and "screen sharing" mean two different things they are increasingly used in interchangeable ways as many of the new tools and online services integrated them both, and companies prefer to use terminologies that make more immediate sense to the end user.
Screen-sharing generally requires abundant amounts of bandwidth and therefore it is best used when all participants are on a rather speedy internet connection.
Key advantages in using screen sharing tools to present materials to others are generally the ease of use and immediacy that this approach provides. On the other hand, screen sharing key limitations are the need for a good Internet connection to achieve effective results.
Screen sharing also does not fare well against Web-touring/Co-browsing and PowerPoint dedicated delivery tools. These generally provide better display and performance while using less bandwidth."
Robin Good's Miniguide is at http://www.kolabora.com/news/2006/10/05/screen_sharing_tools_and_technology.htm