Plaque is a soft, pale yellow, sticky substance that builds up on the surfaces of teeth if you don't brush them regularly. It can cause tooth decay and gum disease. Look at the model to see plaque building up on the teeth. It tends to collect in protected areas such as in between the teeth, and in the grooves on the biting surfaces of back teeth (called fissures).
After a few days of not brushing, plaque can start to make teeth feel rather 'furry', and they can begin to look slightly yellow and sometimes green. If you looked at plaque under a microscope, you would see that it contains lots of tiny bacteria living in a sticky substance.
You can remove plaque yourself using a toothbrush and fluoride toothpaste. You need to take special care over hard to reach areas such as between teeth, and your dentist or hygienist can tell you how to do this. Go to the oral hygiene section of the site for a demonstration on toothbrushing or flossing.