CT scan. A virtual colonoscopy is a ct scan of your colon used in place of a colonoscopy. A colon prep is still required and if polyps are detected then you still need a colonoscopy to remove the polyp. It is also limited in that it can only detect polyps > 5 mm, while the majority of polyps are < 5 mm or flat.
Capsule colonoscopy. Virtual colonoscopy involves swallowing a capsule which then takes multiple photos of the intestinal tract on it's way down the body. Although the results are reliable, they are limited because they don't catch all angles. The gold standard and most reliable method is still a colonoscopy as it is the only way to directly get a view of the intestines, as well as treat and biopsy some lesions.