This is the first of two expository lectures on perverse sheaves and their uses. I plan to discuss the definition and properties of perverse sheaves along with the most important result concerning them: the decomposition theorem of Beilinson, Bernstein and Deligne. The goal of the second lecture will be to understand some of the uses of this result in the context of Lie groups, such as Lusztig's construction of the canonical basis and the geometric Satake correspondence.