Originally Posted by momoese

Yes it's all about glass and that's where you will spend most of your money, or should anyway.

When they talk about fast glass they are simply referring to the max aperture size. Smaller number = bigger opening = more light hitting the film (sensor) equals the ability to use faster shutter speeds and lower ISO to avoid grain (digital noise). It also allows for better control over the DOF (Depth Of Field) Wide open apertures like 2.8 or bigger like 1.4 will produce a very shallow area of focus or DOF. I use this with most of my dog portraits or flower pictures to blur the background making to the subject the center of attention.

In general the bigger the aperture the more money you spend, especially with wide angle or long lenses. So a lot of camera shopping comes down to your budget and what you are looking to use the gear for. You will find better deals on used Nikon/Nikkor glass than Canon. Canon bodies are also a little more pricey. Most people in the know these days say you can't tell the difference in image quality between the two brands using comparable equipment so buy what fits your budget.

Hope that helps to explain fast glass.