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.