Trigger circuit voltage

Old flash units - both studio and hotshoe-mount - used pretty high voltages between the camera and the flash - often from 25 to 250 volts. This is because the flash units were fired by simple switches - electrical contacts. Modern cameras, however, rely on electronic circuitry rather than electric switches. This allows for more flexibility and the possibility for computerization, but the circuits can't withstand high trigger circuit voltages anything above 6 volts, in the case of EOS cameras,...

External battery packs

Most of Canon's high-end flash units have sockets on the side which can accommodate external high-voltage 270 volts battery packs. These packs have two basic functions - they speed up the flash's recycle time between shots to a second or two critical for news or wedding photography and extend the time you can go between changing batteries. They're also useful in cold weather battery performance always drops precipitately at freezing temperatures since you can stuff the pack inside your jacket...

Type A and type B bodies

Type A bodies, with support for E-TTL flash, FEL and FP mode EOS D2000 digital EOS Rebel G 500N New EOS Kiss EOS Rebel 2000 EOS 300, Kiss III EOS IX, IX 7, IX Lite, IX 50 APS EOS Elan 7 E , EOS 30 33 7 EOS 1V EOS D30 and D60 digital EOS 1D and 1Ds digital EOS 3000N EOS Kiss IIIL EOS 300V Rebel Ti Kiss 5 Type B bodies, with support for TTL and A-TTL only EOS 600 series - 600, 620, 630, 650, RT EOS 700, 750, 800 EOS 1 First generation Rebel series - Rebel, Rebel S, EOS 1000 and all 1000 variants,...

TTL and ETTL and EOS digital cameras

All current Canon digital cameras with hotshoes - both the D30, D60, 1D and IDs digital EOS cameras and the non-EOS PowerShot Pro 70 IS, Pro 90 IS, G1 and G2 point and shoot digital cameras - support E-TTL only. Even Canon digital cameras with internal popup flashes are E-TTL only. (though if you want to use flash with a non-EOS camera you should probably check out Kevin Bjorke's page for its limitations. Canon have also written a letter to D30 users concerning proper use of EX flash units)...