You will usually have an online whiteboard where PowerPoint or similar content, documents, pictures, even websites, can be posted and marked-up by participants. There are also usually tools to allow for real-time discussions and conducting online polls or surveys. Keep in mind that most of these services are graphics intensive, utilizing Javascripts and file formats that most likely will not operate as quickly for those on slow Internet connections. It will convert PowerPoint presentations to web pages, and will help you plan your presentation by registering potential attendees, allowing you to conduct polls and surveys. Organizers will receive reports on who attended and how long they stayed.
Any participant can add notes or comments on screen.
In order to share applications, users have to download a small utility, which then allows them to remotely control a program on someone else's desktop. The free account will let you host a conference room of up to three people. More users entail a full account to be set up. CentraNow is a free service that limits you to a 30 minute meeting with a megabyte of content and five participants. Presenters, can, however, switch control of the presentation itself, discussion screen, or survey window to any participant.