An important question to ask when selecting a lens for purchase or for a shoot is: "What is the maximum aperture opening I need?"
The aperture value (f-stop) is the ratio of the focal length to the diameter of the entrance pupil. The number following the "f" in the lens model name refers to the maximum measurement ratio the lens can create.
Here are f-stop examples in full-stop variations, wide (bright/fast) to narrow (dark/slow) from left to right:
f/1.4 | f/2.0 | f/2.8 | f/4.0 | f/5.6 | f/8 | f/11 | f/16
A one-stop difference represents a 1/2x or 2x difference in the amount of light transmitted, and a one-stop change is a big deal.
Because the maximum opening of some lenses does not increase enough with focal length increase to maintain the same maximum aperture ratio, they have a variable max aperture referenced like f/4-5.6. The widest focal length of this lens has an f/4 aperture available, while the telephoto end opens only to a 1-stop-narrower f/5.6.
Typically, a wide maximum aperture lens:
Typically, due to increased lens element diameter, a wide maximum aperture lens has:
When determining how wide your lens aperture needs to be, consider:
How wide of an aperture does one need? That answer varies dramatically for individual scenarios, but if you are shooting indoors without a flash (or with a fill flash only), you will probably want an f/2.8 or wider aperture. If your subject is still, image stabilization can be a substitute for a wide aperture (f/4 should be adequate), but image stabilization does not help stop subject action (though flash as the primary light source can).
If what you are shooting is indoor action (such as sports), you will appreciate an f/2.0 or wider aperture unless your lighting is unusually bright. An f/2.8 lens is often used in these situations, but an ISO setting of 3200 or higher is often required to get close to action-stopping shutter speeds.
An f/4.0 maximum aperture is generally good in medium lighting levels. An f/5.6 maximum aperture requires good lighting or image stabilization unless outdoors before sunset.
If you are shooting landscapes from a tripod, you are likely happy with f/8.0 or f/11.0. That your lens opens wider may be of little importance.
Use what you learned here to help you decide which Canon lens is right for you. Next, go through my specific Canon lens recommendations.