As Bill Brown says, you can think of a lens as your "camera's eyeball."  It receives light and focuses an image.  A Lens is a collection of lenses called elements.  

If your lens isn't simple and inexpensive, then it is made up of separate elements that are aligned along a common axis.  These exist so that there will be a sharper image with fewer aberrations.

Lenses are defined by their focal length:


The focal length of a lens is the distance from the optical center of the lens to the focal plane when the lens is focused on infinity.