Kidney stones are small, hard deposits that form in the kidneys when certain substances in the urine become concentrated. They can vary in size, from tiny grains to larger stones, and can cause severe pain when they move through the urinary tract. Common symptoms include sharp pain in the back or side, blood in the urine, and frequent urination. There are different types of kidney stones, including calcium stones, uric acid stones, and struvite stones, each formed from different substances. Staying hydrated and maintaining a balanced diet can help prevent their formation. If someone experiences symptoms, it's important to consult a doctor for proper diagnosis and treatment.