It's not a silly question at all! Your "water" is actually amniotic fluid, which surrounds the fetus as a protection. It's like a big balloon in which the baby is floating. The "bag of waters" has a membrane around it that is resistant to breaking, and that's why when your water breaks the provider will say that your "membranes ruptured." No one knows why labor begins, but one way that labor starts is when your water breaks spontaneously, and labor begins shortly afterward. This happens about 30 percent of the time, according to Dr. Susan Warhus's book Countdown to Baby. If your water breaks after 37 weeks of pregnancy, it is considered normal. You can feel a big gush of fluid running out of you (this happened to me in a furniture store with my first pregnancy), or you might feel just a little wet. If you think your water has broken, you should always call your provider, for the amniotic sac is the baby's protection, and after your water has broken you need to take precautions to keep the baby safe from infection. In 70 percent of cases, however, your water doesn't actually break until you are in active labor. Often if you're in the hospital in labor, your provider will manually "rupture the membranes" (by using a sort of hook during a vaginal exam) in order to speed up your labor.
If your water breaks before 37 weeks of pregnancy, you have what is called "preterm premature rupture of membranes" (PPROM). That's a bigger problem, for we don't like babies to be born before 37 weeks. This premature rupture is many times due to an infection in the uterus, and it is essential to contact your provider immediately if this happens.