It is a condition also known as 'sensitive gut' or 'spastic colon' or 'nervous colon'. In the US, it affects 20% of people. In Singapore, it affects about 10% of people. IBS tends to affect more women than men. IBS tends to occur most frequently in the 30s and 40s.
No one knows the exact cause of irritable bowel syndrome, but it has been associated with altered bowel motility (gut that moves faster, or slower or even tends get spasms), visceral hypersensitivity (people with IBS tend to be more aware of their gut contractions and movement), psychological factors (stress tends to trigger IBS symptoms and dealing with prolonged IBS symptoms can be stressful), neurotransmitter imbalance, and infection or inflammation of the gut (about 30% of patients report IBS symptoms after a bout of gastroenteritis).
There is no specific test to diagnose IBS. It is mainly a clinical diagnosis after exclusion of certain 'red flag' indicators. There are certain diagnostic criteria that will help your doctor make a diagnosis.