Gallstones are small stones that form in the gallbladder or the bile ducts, the tubes that carry bile to the small intestine. They usually don't cause symptoms unless they block the opening to the gallbladder. If that happens, you may have pain in the upper right part of your belly. You may also have a fever, chills, nausea, or vomiting.
Sometimes a thick material called "sludge" forms instead of stones. This can cause the same problems as gallstones.
If stones or sludge block a duct, treatment is usually needed. If you have a bad attack, or a second attack, you may want to have your gallbladder removed because you are likely to have more attacks.