Probiotics are different strains of helpful bacteria and yeasts that we take as supplements to help populate our digestive tracts with good bugs. A lot of IBDers have an imbalance of good and bad bacteria in their tracts, so taking supplements helps to regain the balance.
There are so many different probiotic brands, and everyone has different experiences with them. It is useful to get recommendations from people on what they take, but if one brand doesn't work for you, don't give up, just try a different type. What I've noticed is that brands that contain a good variety of bacteria strains are more helpful, as well as ones that contain a large quantity of bacteria.
Some brands that have helped me are Florastor, FloraQ2, and VSL#3. Unfortunately all 3 brands are on the expensive side, but they all helped reduce my UC symptoms. Fewer bms, better digestion of foods, less gas, less pain in my gut.
If you go to just about any health food store (Whole Foods, Nature's, Wild Oats, Sunflower Markets, GNC, or a local food co-op), they'll have a decent selection. Just ask the supplements person to point you in the right direction. Something else you can do is to do a search of the HealingWell forums to get ideas. The search function is located just below the site banner...just type in probiotics...you'll find tons of info. you could also ask your doc, although many don't know about pro-bs or believe in their usefulness.
Good luck on the search.