Canon over-took Nikon's dominance in the professional 35mm photography world after they introduced their revolutionary autofocus capabilities of their new EOS system in 1987.
In order to achieve this they decided to abandon their breech-lock manual focus lens mount (Canon FD lenses) and create a new FD-incompatible bayonet mount designed for autofocus lenses, the EF mount.
Canon continued with this mount when they migrated to digital SLRs, while adding a sub-type of this mount for their cropped sensor cameras which it called EF-S as the lenses had a smaller lens circle.
It is likely these dSLRs will be gradually phased out from 2019 onwards with the development of the Canon R full frame and APS-C mirrorless camera system system to replace it - the 1st to go are thought to be the 50mp dSLRs and the 7D series.
The professional Canon lenses are designated “L” lenses and are generally weatherproofed.
most of the L series telephotos are white which may be good if you are an extrovert pro at sports events, but not so good if you don't want to scare everyone around you and draw attention to yourself.
you may not be allowed into sports events with one of these unless you have a pro photographer's permit pass for the event.
Olympus pioneered TTL flash in the late 1970's with their brilliant OM-2 film SLR which was the first to have off-the-film real time flash exposure metering during the exposure.
Canon finally created their version of this in 1986 for the Canon T90 FD-mount film SLR, called Advanced-TTL or A-TTL and, unlike the Olympus TTL system, it also used pre-flashes to determine subject distance and flash exposure, but as this was detected by the flash unit rather than metering off the film, it caused many problems.
Canon replaced A-TTL with Evaluative-TTL or E-TTL which used preflashes and the camera's usual metering system combined with data of subject distance from the selected AF point to determine flash exposure without any off-the-fim metering. This unfortunately created issues when AF was locked on a subject prior to taking the photo.
Canon updated E-TTL to E-TTL II and ceased using the selected AF point, and used lens focus distance data if available (only newer lenses had this capability) and combined this with ambient and pre-flash metered data. Ignores hot spots due to reflections of the flash. Allows AFL and reomposition.
interestingly, the Canon off-camera TTL flash cord is cross-compatible for use on most Olympus and Panasonic cameras as the hotshoe configuration is similar (of course you cannot use a Canon flash in TTL mode on a non-Canon camera and vice versa, but they will work in manual mode). DO NOT use Canon flashes on the new power-enabled hotshoes such as on the Olympus OM-D E-M5 II and later models.
Why buy a Canon dSLR?
There are some excellent reasons to buy a Canon dSLR (or any other dSLR), but if none of these apply, perhaps you should be looking at mirrorless camera systems
you aspire to be a enthusiast or a professional photographer and wish to either:
build up a kit of pro L lenses
use certain lenses only available in Canon eg. 17mm tilt shift, 85mm f/1.2 L, MP-E65mm f/2.8 1-5x macro
you aspire to be a strobist and want to use remote TTL using PocketWizard radio remote controls
you need full camera control with 3rd party utilities (eg. astrophotography)
you already have Canon EF gear
you want to capture fast moving subjects (mirrorless cameras and entry-level dSLRs are not great at this)
you want to buy a full frame dSLR
you need equipment readily serviceable nearly anywhere in the world
you want to look like a pro
Canon dSLR categories
budget cheaply made entry level
generally avoid this category unless you really need a Canon and can't afford anything else
these cameras are generally the worst of all worlds - you are likely to be better off with a mirrorless camera systems or pay the extra $100 or so and get a better dSLR.
HOWEVER, as cameras are now disposable commodities lasting only 3 years or so, buying a cheap camera may not be so silly as long as it does what you need it to do.
eg. Canon 70D - this has a revolutionary sensor introduced in mid 2013 which should give the best AF in Live View of any current camera, although this is yet to be proven, however, being a dSLR you have to use the rear screen on not the viewfinder for Live View mode - a bit of a pity really.
sports enthusiast level
weatherproofed pentaprism dSLR optimised for action/sports photography
1/8000th sec shutter, faster burst rates, faster flash sync and more AF points
N3 connector instead of E3 found in cheaper models
it appears mirrorless fullframe cameras will see the end of this line in 2019
it appears mirrorless fullframe cameras will see the end of this line in 2019
Canon APS-C lens outfit:
if you buy an APS-C cropped sensor Canon dSLR, be aware that the EF-S lenses are limited and generally of consumer standard and not pro quality, and you will not be able to use these on a full frame dSLR if you migrate to this later.
you may want to consider slowly building up a pro L lens kit if you can afford it rather than spend money on lower quality EF-S lenses you may end up just having to resell when you upgrade.
an example APS-C lens kit using EF-S lenses primarily: