Canon Rebels and the 7D have a 1.6 x cropped sensor (the sensor is 1.6 x smaller than "standard" 24x36 mm film or a full-frame DSLR). This means that focal lengths must be multiplied accordingly. The 50 mm on a crop-sensor body has the same angle of view as an 80 mm lens.
The 50 mm f/1.8 was a workhorse in the days of film, as shown by its remarkably low price. The f/1.4 is much better, much faster, has more pleasing bokeh, but costs three times the price of the f/1.8. The 50 mm f/1.2 L is over ten times the price of the f/1.8, but I don't know whether it's worth it. These lenses are "fast", in other words, at maximum aperture there's a lot of light coming in, which makes them ideal for low-light situations such as weddings (dark churches), concerts (dark stages), etc. The drawback of using a "nifty-fifty" wide open at, say f/1.4 is that image quality can be iffy (distortion, softness, chromatic aberration, vignetting and so on) and the depth of field is by definition narrow.
This post shows just how extreme image softness can be on a Sigma 50 mm f/1.4 lens - admittedly on a 21 MP full-frame camera that punishes below-par lenses mercilessly. The effects are not quite as pronounced when using a crop-sensor body.
The popularity of 50 mm focal lengths (real, not multiplied by the crop sensor factor) stems from the fact that they equate to what one human eye sees. For a cropped sensor body, something in the region of 30-35 mm does the trick. To my knowledge, Canon doesn't make a fast 30 mm lens, but Nikon and Sigma do. Sigma's 30 mm f/1.4 is a very good, but expensive, lens for crop-sensor Canon bodies. It is, however, rather large and bulky on the smaller Canon bodies.
All these lenses are "primes" (no zoom), which are usually regarded as offering better image quality than zoom lenses covering the standard 24-70 mm (full-frame / film equivalent) focal lengths.
