Anamorphic lenses are specialty tools that affect how images get projected onto the camera sensor. They were primarily created so that a wider range of aspect ratios could fit within a standard film frame, but since then, cinematographers have become accustomed to their unique look.
Two classes of lenses are typically used in production: spherical and anamorphic. Spherical are more common and are the assumed lens type unless specified otherwise. Spherical lenses project images onto the sensor without affecting their aspect ratio. Anamorphic lenses, on the other hand, project a version of the image that is compressed along the longer dimension (usually by a factor of two). Anamorphic lenses therefore require subsequent stretching, in post-production or at the projector, in order to be properly displayed.
See full details and examples of Anamorphic Lenses in the RED 101 Article: Understanding Anamorphic Lenses