Colitis is simply bowel inflammation. A diagnosis of colitis means they saw inflammation.
Over long periods colitis might indicate inflammatory bowel disease, two forms of which are ulcerative colitis or crohns disease. Ulcerative colitis is usually a specific autoimmune disorder where your immune system sees your colon as a hostile entity and attacks it. Crohns disease is a closely related disease where it is thought the immune system is targeting normal gut bacteria erroneously. Crohns disease also commonly also targets the ileum, the end of the small intestine.