On the outside, we think of the flash as providing a momentary burst of light ... and it does that. But there are a lot of nuances that result in interesting capabilities and features that separate flashes.
A completely manual flash is typically the least expensive. But it's up to you to determine how to control the flash power. Most flashes (even many non-expensive units) have at least some capability to automate this so it's often not necessary for the photographer to have to learn to control a flash manually... but still helpful.
As a beginner, you *probably* want a flash that supports the Canon E-TTL system. This automates how the camera & flash work together to compute the correct amount of power for a shot. Usually (but not always) it will do a good job. This avoids you having to know how to control flash power (something you should learn anyway, but having some automation does simplify things).
You definitely want a flash that allows the flash head to be tilted and rotated (so you can bounce and feather the light).
You also want a flash that allows you to focus the reflector so you can have a tight or broad beam of light.
You also want a flash that can be used and trigger off-camera as a "slave" in an E-TTL system. Canon's latest flashes have radio technology built-in (but the camera does NOT have a radio transmitter built-in so you'd still need a way to trigger the off-camera flash ... either a 2nd flash or a dedicated radio controller can do this). Most flashes that can work off-camera can be trigger via "optical" trigger. Radio doesn't require a line of sight.
There are many other features...
High speed sync mode (allows you to shoot at speeds faster than the maximum flash-sync speed)
2nd curtain sync mode (good for moving subjects in long exposure shots)
Multi-strobic mode (I've never used this... except to play ... it allows the flash to pulse at a timed rate you control during long exposure shots (e.g. a moving subject shows up in multiple positions within a single shot)
But one of the biggest features is the power. How much light will the flash be able to throw on a subject at a distance.
This power is rated using something called a "guide number". Guide number could measured in feet or in meters but most use meters these days (Canon uses meters on their flashes). For Canon brand (and ONLY for Canon brand... 3rd party flashes that are compatible don't do this), the guide number is part of the flash's model number. For example, the 600EX has a guide number of 60 meters. The 430EX has a guide number of 43 meters. The 320EX has a guide number of 32 meters (e.g. just remove the "0EX" from the model number and the remaining digits are your guide number).
So what's a guide number?
It's the distance at which the flash can adequately illuminate a subject ASSUMING you are using ISO 100 and the lens aperture of f/1.0.
The ISO 100 is no problem... but what's up with f/1.0? Who has an f/1.0 lens (although some have been made). The reason for f/1.0 is that is a baseline that makes the math easy. All you do is divide the guide number by the ACTUAL f-stop you plan to use ... and that's your distance (but it would only be that easy if the Guide Number system was based on f/1.).
So if you plan to use f/5.6, you divide the guide number by 5.6. E.g. suppose you have a Canon 430EX III-RT flash (Guide number is 43 meters). 43 meter is 141 feet. Divide that by 5.6 and you get about 25 feet. That means the flash can adequately illuminate a subject 25 feet away if you use f/5.6 (at ISO 100). Pretty easy once you know the rule.
You might think 25 feet sounds like a lot...
BUT... you may be using a flash modifier... a soft box, a shoot-through umbrella. You might be reflecting the flash (bouncing it) off another surface (such as the ceiling or wall). All those things "eat" light (probably at least half the light). Bouncing means it also has to travel a greater distance and if it bounces off a "scattering" surface (diffusion) than the light is considerably more spread out (less light lands on YOUR subject because more of it scatters off in other directions that miss your subject).
For this reason, it's nice to have a powerful flash when you plan to use it with flash modifiers.
