The Canon speedlite system is a source of confusion to many new (and quite a few old!) Canon EOS users. This has been made somewhat worse with the introduction of DSLRs as they are not really compatible with early "EZ" series Speedlites. DSLRs require newer "EX" Series speedlites in order to provide a reasonable degree of automation and allow automatic exposure.
"EX" series speedlites allow the use of E-TTL (and E-TTL II) which are flash metering systems which use a preflash and the camera's multi-zone evaluative metering system to determine optimum flash exposure. Earlier "EZ" series speedlites did not use a pre-flash and determined flash exposure by measuring the amount of light being reflected from the film surface onto special flash exposure sensors during exposure. [Note that "EX" series speedlites default to the older "A-TTL" mode of the "EZ" series when used with older EOS bodies which don't support E-TTL.] Since DSLRs have no film and they are anti-reflection coated, this metering system can't be used and so "EZ" series speedlites are incompatible with auto exposure on Canon EOS DSLRs. They can be used in a fully manual mode, i.e. basing the aperture setting on the camera on the guide number of the speedlite, the ISO setting and the subject distance.
ETTL-II is very similar to E-TTL, except it uses a slightly different algorithm to calculate flash exposure and it can also factor in the distance to the subject when required. You can read a bit more about it here - ETTL II
"EX" series speedlites do provide the pre-flash which E-TTL (and E-TTL II) metering requires. TAs of August 2008, there are currently three "EX" series speedlites in production (plus a couple of specialized macro ringlites). They are the 580EX II, the 430EX II and the 220EX (with the 420EX and 550EX discontinued and the 430EX soon to be phased out). They differ both in flash power and in features. They all support FP high speed flash sync (at reduced power). There are also two specialized macro speedlites, the MR-14EX and the MT-24EX.
Here's a table which summarizes some of the differences between the 580EX II, the 430EX and the 220EX Speedlites:
|Wireless:||No||Slave||Controller and Slave|
|AF-assist Beam:||Yes||Yes, covers all focus points in EOS cameras up to 9 AF points||Yes, linked to multi-point AF system of compatible cameras|
|Guide No. (ISO 100, in meters) Zoom/Guide:||28/22||24/23; 28/26; 35/32; 50/35; 70/38; 105/43||14/15; 24/28; 28/30; 35/36; 50/42; 70/50; 80/53; 105/58|
|Flash Covering Setting Method:||Fixed condenser lens||Fixed condenser lens with internal zoom||Fixed condenser lens with internal zoom|
|Flash Coverage Setting System:||Fixed||1. Autozoom (zooms automatically to match the lens focal length)
2. Manual Zoom (with zoom button)
3. Wide Panel
|1. Autozoom (zooms automatically to match the lens focal length)
2. Manual Zoom (with zoom button)
3. Wide Panel
|Image Size Zoom Control:||None||With cameras compatible with auto zoom, it zooms automatically to match the camera's image size||With cameras compatible with auto zoom, it zooms automatically to match the camera's image size|
|Color Temperature Control||None||automatically transfers colour temperature information to recent models of digital SLR cameras||automatically transfers colour temperature information to recent models of digital SLR cameras|
|Zooming Speed:||None||Approx 0.4 sec||Approx. 0.6 sec. (24-105mm)|
|Zoom Position Indicator:||None||numerals on the LCD's focal length display||numerals on the LCD's focal length display|
|Bounce Direction:||None||Up: 0-90 degrees; Left: 0-180 degrees; Right: 0-90 degrees||Up: 0-90 degrees; Down: 0-7 degrees; Left: 0-180 degrees; Right: 0-180 degrees|
|Bounce Position Indication:||None||Markings on back of flash head||LCD indicates 7 degree bounce down only|
|Manual Power Setting||None||1/1 to 1/64||1/1 to 1/128|
|Exposure Control Modes:||E-TTL II, E-TTL, TTL||E-TTL II, E-TTL, TTL||E-TTL II, E-TTL, TTL|
|Flash Modes:||Normal Flash, High sync speed, Test firing||Normal Flash, High sync speed, Test firing, Modeling flash||Normal Flash, Quick Flash, High sync speed, Stroboscopic Flash, Test firing, Modelling flash|
|Flash Range - ISO 100 50/1.4||0.7m -15.7m||0.7-24.3m||0.5m - 30m|
|Custom Functions:||None||6, user selectable||14, user selectable|
|Other Features||Weather Sealed, metal mounting foot.|
|Internal Power Source||4x AA||4 xAA||4 x AA|
|External Power Source Option||No||No||Yes|
|Dimensions (W x H x D) mm:||65.1 x 92 x 62||72 x 122 x 101||76 x 134 x 114|
|Weight (excluding batteries):||160g||330g||375g|
If I was a typical amateur photographer who was looking for a new speedlite to go with a new DSLR, I'd probably go for the 430EX, unless I was really serious about getting into multiple flash photography. It has most of the features that average photographer is likely to need for single flash work. If you want to use multiple flash units, then the master wireless control features of the 580EX II are required but you'll still need a 430EX (or another 580EX II) as your wireless slave. The new 430EX II adds a number of features to the 430EX, but also adds $90 to the price ($330). Since you can get a 580EX II for under $410, I think I'd be tempted to get the 580EX II over the 430EX II, at least until the price on the 430EX II drops a little. The 430EX at around $240 is probably still the best value if you don't need a master wireless controller.