Camera lenses are cryptic enough I feel as though they need their own section.
A set of numbers can be found on all lenses (at least that I have used). For example, on the basic lens for my Sony α, the numbers look something like
DT 3.5-5.6/18-70 Ø55 0.38m/1.3ft MACRO
There may be letters (such as the DT above) that indicate various vendor-specific things about your lens. We will ignore them. Below is an abstract of the basic numbers.
Focal Length
Focal length is a critical consideration when purchasing a new lens. The focal length is the distance between the front of the lens and the camera's exposure plate. For digital cameras, the exposure plate is the sensor, while a traditional camera's exposure plate is covered with a frame of actual, honest-to-God film. For the example above, the focal length is 18mm to 70mm.
The smaller the focal length, the wider the viewing area will appear (also known as wide angle). Also, the smaller the focal length, the shallower the depth of focus in your photographs. The manufacturer of your lens should provide specifications for its depth of focus and other characteristics. Sometimes, the focal length of a lens is indicated by a single number. A lens of this type is said to be a prime lens, meaning that its focal depth can not be adjusted.
f-stop
The decimal numbers (e.g. 3.5-5.6) indicate the maximum size of the aperture (f-stop) at both ends of the lens' focal length. For the example, at a focal depth of 18mm, the lens' maximum aperture size is 3.5. At a focal depth of 70mm, the maximum aperture size is 5.6. Note: the smaller the f-stop, the larger the aperture.
Filter Size
Most lenses come with a threaded ring onto which may be screwed filters for various tasks. For example, with my Sony α I always employ a UV filter to help protect the exposure plate from UV. The Ø55 simply indicates the diameter, in millimeters, of the threaded circle provided for attaching filters to the camera lens.
Macro Lenses
The 0.38m/1.3ft MACRO indicates the distance from the objective the lense must be when taking a macro (close up) photograph.