Your subject

• Are you shooting portraits? Trying to get into sports photography? Capturing landscapes? These very different things require very different lenses. If you aren't looking for professional quality and sharpness, Canon makes several "all purpose" lenses that still can produce impressive images without juggling lenses and with much reduced weight.
  
  

Focal length

• Focal length is measured in millimeters and represents the magnification (or demagnification) of a lens. The rule of thumb is that every 50mm represents 1x magnification so that a 100mm lens will have 2x magnification and a 400mm lens will have 8x magnification.
  
  Most "wide-angle" lenses have a focal length of less than 50mm. While lenses used for portraits usually have a focal length between 50mm and 150mm. Telephoto lenses used for sports or nature begin at 200mm and go up from there.
  
  Taking portraits with a 400mm telephoto is almost impossible unless you intend on being about 50 yards away from your subject, and shooting wildlife with a wide-angle lens almost guarantees your beautiful bird will show up like a speck in your photo.
  
  

F/stop

• F/stop of aperture means the amount of light a lens allows on to the exposure medium (film or a digital sensor.) It is easier to think of aperture as a fraction: 1/1 would be the same as having no lens at all, where as 1/33 means much less light getting in.
  
  If you plan on shooting slow moving things under bright light then aperture is less important, and an aperture of f/5.6 should be fine. But capturing a bird in flight, a portrait at sunset or a highlight dunk will require a lot of light and a BIG aperture.
  
  Now for the bad news, aperture is one of the biggest cost factors in lenses. Big aperture means big money. As of 2009, a Canon 400mm lens with an aperture of f/5.6 costs $1,200, but the same focal length with a f/2.8 cashes in at $7,200!
  
  That 400mm f/2.8, incidentally, is what Canon photographers use to shoot football.
  
  

Putting it all together

• So your mom wants a picture of the mountains - a wide-angle lens with a small aperture will bring them all in.
  
  If your mom visits and wants a picture of herself with the mountains in the background, time to break out a lens with medium focal length and medium aperture.
  
  If your mom spots the antelope running with the mountains in the background at sunset with the mountains in the background it's time to get a longer focal length and a wide aperture.