The creatures use their limbs to wander around their virtual world. The maximum number of creatures that can appear is 25. In the beginning, not all of the creatures will be capable of moving around, so you'll need to wait for them to evolve to see any real action.
The screensaver saves the creature evolutions when you close it, and reloads them when it starts up again. Days or even weeks may pass before you have creatures that manage to reach a noticeable degree of locomotion.
You can restart the virtual world from zero whenever you want from the screensaver's control panel.