Most polyps are protrusions from the lining of the intestine.
Polypoid polyps look like a mushroom, but flop around inside the intestine because they are attached to the lining of the colon by a thin stalk.
Sessile polyps do not have a stalk, and are attached to the lining by a broad base.
Flat polyps are the least common type of colon polyp, and are flat or even slightly depressed. These may be difficult to identify because they are not as prominent as polypoid or sessile polyps with the commonly-available methods of diagnosing polyps.