Combined Oral Contraceptives (COCs) are a form of hormonal birth control that combines estrogen and progestin to prevent ovulation, thicken cervical mucus to block sperm, and thin the lining of the uterus to prevent implantation. They are highly effective when used correctly and offer additional benefits such as reduced menstrual cramps and acne, but they also carry risks like increased chances of blood clots and require a prescription.