You can either create your own diagrams from scratch, hand-drawing them with the mouse, or import an Excel file, which you can then edit and modify however you like. Also, once you have finished a diagram you can export it in any format you like such as PNG, JPG, SVG, PDF, SWF, etc.
To draw your diagrams, you are given a variety of items, ranging from basic shapes to small representations of computers, people and other objects. Bringing everything together is as simple as dragging the mouse from one side of the screen to another, while more complicated patterns only require a few additional mouse movements.
yEd is a really interesting tool due to its ease of use and its scope. If you have to create visual diagrams regularly, don't hesitate to take a closer look at this app.