A lens is a piece of specially designed glass that projects an image onto a camera sensor. In which the field of view is mattered most.
See this Link to learn about Field of view in different cameras and their sensors.
More on lenses for dummies found here
Full Frame and Cropped Sensor
Canon Prime Lens Comparison with 5D from GEBBS on Vimeo.
Compare 5D & 7D with Prime Lenses
Carl Zeiss Lenses
What would you use for CINEMA - Zoom or Prime ?