Cranberry

Cranberry has been used for thousands of years as a food, although a lot of people will not eat it very often because it is often eaten as cranberry sauce, which is usually only eaten on special occasions. Cranberry juice is quite commonly drunk, as well.
Cranberry contains large amounts of various nutrients, including vitamin C and proanthocyanadins - the latter provide the berries' deep red colour. This means that cranberries have all the same health benefits as most other deep-coloured fruits; they help the condition of vein and artery walls, the skin and various other connective tissues, and help protect against cancer and heart disease. This is largely because the vitamin C in cranberries is antioxidant and the proanthocyanadins are more so.
Cranberry, however, has more specific benefits - which are the usual reason why cranberry is taken as a supplement. Cranberry's specific blend of proanthocyanidins, together with the related compound hippuric acid, stop bacteria forming biofilms. This matters because the chief cause of cystitis, from which probably most women suffer at some time (men don't get it anything like as much), is bacteria colonising the wall of the bladder and urethra. The main one is E. coli.
Cranberry extract therefore gets rid of cystitis by making it impossible for the chief cause of the problem to stick to the bladder walls. Cranberry's vitamin C will also undoubtedly speed up the healing process once the bacteria have gone.
The usual cautions apply here. Cranberry should not be used as a substitute for the proper treatment in severe infection; bladder infections can spread to the kidneys and when they do they become a severe problem. Also, the proanthocyanadins in cranberry are probably anticoagulant - and therefore cranberry should not be taken, at least in concentrated form, if you are on anticoagulant drugs such as warfarin.
Cranberry, nevertheless, may well help cystitis turning into a severe infection in the first place, and regular use of cranberry may well help prevent cystitis.