The infrared images are effectively showing the temperature of the cloud or the ground. The images are usually prepared in such a way that cold surfaces appear white, and warm ones darker. The infrared satellite images are updated four times a day. A combination of visible and infrared images is very useful, if both images are bright in a particular area there is likely to be thick, high cloud. If the visible image shows areas of bright cloud but the infrared image in the same area is dark, then there is low cloud or perhaps fog.
Animated images show the movements of clouds and therefore of the associated systems.
The interpretation pages describe satellites in more detail.