Ulcers are open sores or erosions that develop on the skin or mucous membranes, where tissue has been damaged and fails to heal normally, often appearing as shallow or deep craters with a red, raw base that may be covered by yellowish-white debris, pus, or a scab. Common types include peptic ulcers (in the stomach or duodenum lining, frequently linked to H. pylori infection or NSAID use, causing burning upper abdominal pain), mouth ulcers (canker sores: small, painful white/yellow spots with red borders inside the cheeks, lips, or tongue), venous leg ulcers (shallow, irregular sores on the lower legs from poor vein circulation, often with swelling and discoloration), arterial ulcers (deep, punched-out sores on toes/feet/ankles from inadequate blood flow, typically very painful), pressure ulcers (bedsores: from prolonged pressure on skin over bony areas in immobile individuals, progressing from red patches to open wounds), and diabetic foot ulcers (neuropathic sores on feet due to poor sensation and circulation). Symptoms generally involve pain (varying by location and depth), discharge, odor, swelling, or surrounding redness/inflammation; some (like certain peptic or chronic ulcers) may be relatively painless if nerves are damaged.