If you bought a DSLR with the normal kit lens, one of your first lens wishes might be a telephoto zoom for a bit longer reach. An attractive range is 70mm to 300mm, as evidenced by Canon's own three different models (along with an affordable 75-300mm lens). This page will discuss the EF 70-300mm f/4-5.6 IS USM, current retail $649.99.
There is certainly an excitement in having 300mm reach at your disposal for the first time. Whether you're a birder or out to capture other long-range critters (like sporting events), you'll probably be more interested in the 300mm part of this lens, rather than the 70mm end. But would you believe that the maximum 300mm setting may not be the best? There seems to be a "sweet spot" on the long end, beyond which you should not go. For my own specimen, I find that it helps to back it off just a bit to around 280mm.
This lens is surprisingly light and easy to carry all day (as opposed to the L series version, almost twice the weight).
Note: Canon also has the DO (diffractive optics) variant of this lens with image stabilization for $1399.00, and an L series entry at $1349.00.
This telephoto lens was the first lens that I ordered after buying the Rebel XT and 18-55mm kit lens. All of my reading suggested that this would be a fine telephoto zoom for my purposes. Then again, this lens was the reason behind my Real World Lens Tests. When I started taking photos with this lens, I was frustrated to find that I wasn't getting the beautiful, tourist-postcard images that I'd seen during my research online. It dawned on me that digital post-processing was at least as important as digital photography. The osprey sitting atop this page has been cropped, sharpened, color enhanced, with some mild manipulation in light. The bird was a good 80 feet from the camera (handheld). Still, more often than not, I would wish for more clarity, sharper details from this lens.
On the barrel are switches to choose from two stabilizer modes and to turn stabilizer on or off, a switch for auto/manual focus, and a lens lock (without which the lens slides in and out fairly easily).
The minimum focus distance in this case is just about five feet from lens to subject. You won't be using this one for close-ups. But what I discovered when walking around with a telephoto zoom lens is that my eyes were alway looking off to the distance. Basically, what I "see" is in part determined by which lens I was carrying at the time.
f/13, 0.6sec., ISO200, 250mm at a distance of about 75 feet
Following are a few of my earliest photos with this lens, from back in 2006
f/6.3, 1/160, ISO200, 75mm
f/5.6, 1/640, ISO200, 70mm
f/5.6, 1/500, ISO200, 70mm