You might ask, "why Canon, what about Nikon?". This is a perfectly reasonable question. Both Nikon and Canon make superb cameras and are very much in the same league. At some point you pays your money and makes your choice. If you are serious, you make an investment in lenses and that tends to lock you in to one or the other. Sometimes I see a person selling all of their Canon or Nikon gear to switch from one to the other -- I think this is chasing the wind.
Some people will argue that Canon has a better selection of lenses, and maybe this is true. Before accepting this as a blanket statement, you should get some idea of which lenses are important to you and see what each offers. Realize though that your needs and desires can change and mature.
There are of course certain people who want to invest energy in arguing that one make or the other is superior, but this is surprisingly rare. Most serious photographers want to make images and find that either maker offers superb tools for doing so. At any point in time one maker may have an edge on the other, and then later the situation will reverse. Competition is a wonderful thing.
I chose Canon largely because a number of people I knew and respected were using Canon gear, and I have had few regrets. I discuss Canon because that is what I know and use.
You also might ask, "what about point and shoot cameras, why spend all that money for a DSLR". The answer is that you get a DSLR because of the lenses it gives you access to. There is certainly a place in the world for point and shoot cameras, both super compact snapshot cameras, and high end point and shoots like the Canon G10 and G11.
There is an amazing and clever condensation of information in this wikipedia page:
These Canon "white papers" are interesting and informative. They capture Canon's official mindset at specific points in time. Anyone owning one of the cameras discussed will be well served to read the relevant white paper.
I first put this page together back in 2006 when I was new to all of this and was contemplating getting my first DSLR (which ended up being a Canon 20D). Since then there have been a lot of changes, and it is time for an update. From my point of view Canon has 4 lines of digital cameras:
The "rebel" cameras are well priced, have 1.6 crop APS-C sensors, light plastic bodies, and have a mirror rather than a pentaprism.
The "prosumer" cameras (cameras with 2 digit model numbers like the 50D) are cameras with 1.6 crop sensors, heavier bodies, and a pentaprism. Starting with the 60D, Canon seemed to change their philosophy with respect to these cameras, making them less interesting to serious photographers and more appealing to the well heeled average amateur. Interestingly, just at this juncture Canon introduced the 7D, which was the obvious choice for the serious amateur that wanted to stick with the APS-C sensor.
The 5D series offers a full frame sensor in "normal" camera body. The 6D has been added to this group as a "bargain priced" option; sort of.
The 7D belongs in a class of its own, despite being a 1.6 crop camera.
The 1D cameras are all big heavy fully weather sealed units -- and quite expensive.
What kind of photography do you want to do?
How important is photography to you?
How will you present your final images?
Are you interested in shooting video?
How much money can you afford to spend?
Also consider that you will most likely own several bodies during your photographic career, but lenses do not become obsolete. Lenses remain useful and hold their value, whereas bodies become obsolete, loose their value, and get replaced. The choice of a camera body is just one aspect of a photographic budget.
If you shoot landscapes, a camera like the 5D with a full frame sensor will let your wide angle lenses be as wide as they can be. (But the 10-22 S series lens on a 1.6 crop camera solves this problem in a different way.)
If you shoot wildlife or sports, you should consider a 7D or a 1D if you can afford it. You will benefit from a fast autofocus system and rapid frame rate, which are both pretty much meaningless to a landscape photographer.
If you intend to do something like backpacking with your camera, you would want to consider the issue of weight. You might choose a rebel (or a 5D over a 1D).
Ansel Adams once answered the question of what camera he preferred with the response "the biggest one I can carry". He was making big prints and doing landscape photography.
People look for one number to judge a camera by, and lean towards the camera with the most megapixels. This can certainly be important if you intend to make large prints. An important issue for other people is low noise at high ISO settings. Megapixels are important, but they aren't everything. Large files from big sensors can be a nuisance if you don't need all that resolution.
I recommend going to a store and handling different cameras you are interested in. When I was deciding between a rebel and a 20D, I found it very useful to go to a store and handle both cameras. The rebel felt awkwardly small in my big hands, and the ergonomics of the 20D were a big factor in my choice.
Tom's Digital Photography Info / tom@mmto.org