On the front of most lenses is text and numbers these describe the features and capabilities of the particular lens.

In the picture below the generic information found on most lenses has been highlighted.

The other information printed on the lens is the makers special attributes and features of the lens plus the makers name.

Focal length
The Focal length of a lens in a camera is the distance measured from the optical centre of the lens to the image sensor. On a zoom this is adjustable, a Prime lens is preset to a single value.

Maximum Aperture
A fast lens would have a value of f2.8 or higher the bigger the number the slower the lens.

Filter Thread Size
This is the size of the screw thread on the front of the lens and is used to fit mainly filters.