Zoom lenses

Why carry around a whole bag of fixed focal length ("prime") lenses when you could just buy a Tamron 28-300 zoom lens for less than $400? With a twist of a ring, the Tamron will give you any focal length from 28mm to 300mm. The only problem with this idea is that, sadly, the laws of physics and common sense have not been repealed.
Photographic lenses in general are not very good. They only appear to be good because people very seldom enlarge or closely inspect images. Camera lenses are subject to many kinds of distortion, all of which are more difficult to reduce in a zoom lens design. Furthermore, zoom lenses tend to be slower (admit less light) than prime lenses. This forces the photographer into using flash and/or a tripod.
Does that mean you shouldn't buy a zoom lens? Absolutely not. The average Canon EOS photographer will own three beautiful zoom lenses: Canon EF 16-35mm f/2.8L USM (review), Canon EF 24-70mm f/2.8L USM (review), Canon EF 70-200mm f/2.8L IS USM (review). These are a great convenience for the lazy and/or pressed-for-time photographer. However, none of these are as good as prime lenses in their focal length range. Each of these zooms costs over $1000, so they won't help you out if you don't like the prices of the prime lenses.
If you can only have one lens, the Canon EF 24-70mm f/2.8L USM is probably better than the Canon EF 50mm f/1.8 II, $93 (review). But the 50/1.8 is better than cheaper mid-range zooms.