Ulcerative colitis belongs to a group of conditions known as inflammatory bowel diseases, or IBD. Ulcerative colitis causes inflammation of the innermost layer, the mucosa, of the colon (large intestine). Ulcerative colitis causes small sores, or ulcers, to form, which produce pus and mucous. 

The most common symptoms include frequent bloody bowel movements with mucus, a frequent need for bowel movements, abdominal pain, tiredness, gas, and weight loss. 

References:

1. https://www.crohnscolitisfoundation.org/what-is-ulcerative-colitis/overview