What are the rotating cylinders in monastries, and why are they there?

Sorry, as I don't know the right term for them. But, I have noticed these huge copper cylinders(when I went to visit a temple in Dharmashala), which are kept like 5-6 in a row. They have engravings (probably in some script) on them, and they are rotated by the people who are walking past them. So, what are they? And what are the engravings on them? And why people are rotating them?