16 - 35mm? 50mm? 70 - 200mm? What do these number mean? It's actually not as difficult as it may seem. First, let me explain some general terms you may encounter when looking at a lens online.
Angle of View - Amount of a scene that a lens can take in, measured in degrees. A telephoto lens will have a small angle of view (29°, 18°, 5°, etc.) while a wide angle lens will be much larger (43°, 75°, 180°, etc.).
Full Frame Sensor - A 35mm format (standard for the industry) image sensor. Allows a camera to capture the full angle of view offered by a lens as long as it was made for a full frame camera
Crop Sensor - A sensor that is smaller than the standard 35mm (full frame) sensor. Images produced from a crop sensor are equivalent to the middle being cropped out of a full frame sensor image.
35mm Equivalent Focal Length - a term used for crop sensor cameras to express the focal length as if the camera had a full frame sensor.
So what exactly is Focal Length? By definition it's the distance between the optical center of a lens and the image sensor when the subject is in focus, usually stated in millimeters. Got it? Good! However, for those of you still scratching your head, let me explain a little further.
A standard lens for a full frame sensor camera is 50mm since it's the closest perspective to the human eye. Anything less than 50mm is considered wide-angle while anything over is known as telephoto.
What about crop sensor cameras? Most websites will have the 35mm (full frame) Equivalent Focal Length. If not, look for a crop factor such as Canon's x1.6 or Nikon's x1.5. Now, if the lens has a 50mm Focal Length, it will be closer to 80mm on a Canon or 75mm for a Nikon.
Here's a great chart from Digital Camera World in case my ramblings made no sense. Click to see it larger.