Photo 101: Lenses

I can say with certainty that the lens is the most important piece of equipment in your camera bag. Your digital camera body is only temporary. You will upgrade this as technology gets better. Kind of like your computer, you should upgrade about once every 2 years. Your lenses should not be purchased based on replacement. These are your gems. They will become trusted friends in a life full of photographs, so choose wisely, not cheaply. It is not the camera that makes a great photo, it is the lens (and the eye behind the lens, of course!) There are several factors that come into play when choosing a good quality lens.
1. The Glass
The glass must be of the highest quality to render the best clarity in your imagery.
For the best quality in glass you will look for the following: Nikon uses NIKKOR glass and Canon uses EF glass or Canon L Glass (the "L" stands for Luxury.) The optics in the EF L Series Glass (any Canon lens with a red ring around the barrel) are Canon's best quality glass.
2. The Build
You want a lens that is not going to break on you. You will notice instantly the difference in weight between more expensive and less expensive lenses. Metal construction will outlast plastic. With the better lenses you will also want to make sure it is weather sealed.
3. The Mechanics
The zoom and focus rings should be smooth and feel good in your hand.
4. Image Stabilization and Vibration Reduction
Look for IS (Image Stabilization) for Canon Lenses and VR (Vibration Reduction) for Nikon Lenses. This allows you to shoot handheld up to three stops slower than normal.

Okay... so, all that said, let's get down to the dirt! What lens should you buy? The question is always asked: "If you could only have one lens, what would it be?"
The short answer: Canon EF 24-105mm f/4 L IS USM
The long answer: Why that lens and not the Canon EF 24-70mm f2.8 L USM?
Many photographers, and sales clerks, will tell you to go with the 24-70. It is faster by one stop at f2.8 rather than the 24-105's f4.0. It is bigger, heavier, is more expensive (only by about $150), and looks meaner.
MY Answer based on MY usage of both lenses: The 24-70 does not have image stabilization (IS) and the clarity of your imagery is not as great. Bryan Carnathon from The-Digital-Picture.com states: "Very little contrast and saturation post-processing is necessary on most shots taken under good lighting." The operative term being "under good lighting." What I have found is under slightly sub par lighting the contrast and clarity are not as good as with the 24-105 but when stopped down to 4.0 they look very similar. Sooooo, why not extend your range by 35mm, gain 3 stops with the IS, and save some cash by going with the 24-105.

Other lenses to consider:
Canon EF 50mm 1.4 USM
There are three 50mm lenses in the Canon line: 1.8. 1.4, and 1.2
The 1.8 is only $100 and, in my opinion is not worth it. It only has a 5 blade aperture and it's build makes it a throw away lens that will break the first chance it gets.
For just a couple hundred dollars more (around $300) you can get into a better build with 8 blade 1.4 lens. You will see a difference between the 1.4 and the 1.2 (also 8 blade) but it is not worth the extra $1000 (around $1,300)unless you are a professional photographer.