The two primary ways this is done is via whiteboards and application sharing. Whiteboards are the electronic equivalent of a blackboard, letting you copy parts of your application into a whiteboard window that lets you draw and annotate directly on top of it. For example, you might copy some cells from your spreadsheet into the whiteboard and use the pointer to call attention to certain figures. You can use the yellow marker, which can also usually be changed to any color, and circle some cells, draw arrows and so forth. As each user marks the whiteboard, it simultaneously appears on the other screens.
Application sharing is a lot like remote control.
In fact, it's really identical to it if one user can work in the other user's computer without restriction. In its simplest form, one user runs the application, say a charting program, and the other users simply watch the chart being built in their viewing windows. This is technically application viewing, not application sharing. Application sharing allows different users to take control of the application. Suppose user A launches an application and does a little work in it.