When the first camera with evaluative autoflash metering was launched, EX-series Speedlites were introduced to make use of the E-TTL metering. These EX-series Speedlites are also compatible with TTL flash metering, so will operate with all EOS models.
All the new film cameras offering E-TTL metering retain the flash sensors for A-TTL metering, so are also compatible with EZ-series Speedlites.
When Canon introduced digital EOS models, they omitted the flash sensors in the base of the camera (the sensor reflects light differently to film). This means that EZ-series Speedlites can only be used in manual mode with EOS digital cameras.
Canon updated E-TTL flash metering in 2005. E-TTL II is a camera feature, not a Speedlite feature, though it only operates with EX-series Speedlites.
Here is a short guide to compatibility:
Type A cameras: E-TTL autoflash metering is only supported by EOS cameras introduced at the same time or later than the EX-series Speedlites. These models allow the viewfinder sensors to be used for both ambient and flash readings. These cameras, called Type A, include the EOS 1V, 3, 30, 30V. 33, 33V, 300, 300V, 300X, 3000N, 3000V, 50E, 50, 500N, IX, IX7 and all EOS digital cameras. Type A film cameras also include sensors that can read flash illumination off the film during an exposure, so EZ-series Speedlites can be used with A-TTL autoexposure.
Type A-II cameras: There is a sub-group of Type A cameras which enables E-TTL II flash metering with EX-series Speedlites. These include the EOS 30V, 33V, 300X, 1D Mark II, 1D Mark II N, 1D Mark III, 1Ds Mark III, 5D, 5D Mark II, 20D, 20Da, 30D, 40D, 50D, 350D, 400D, 450D and 1000D.
Type B cameras: Other EOS film models are known as Type B cameras and can be used with EZ-series Speedlites in A-TTL (or TTL) mode or EX-series Speedlites in TTL mode.
This may seem complicated, but if you are using an EOS digital camera all you really need to know is that it is only compatible with EX-series Speedlites.
Autoflash means that a flash gun gives just the right amount of light for correct exposure. It could do this by altering the power of the flash, increasing or decreasing the brightness as you change the distance of the camera from the subject. But this does not happen. Instead, Speedlites increase or decrease the duration of the flash as the subject distance changes. The brightness of the flash remains constant. However, even a long Speedlite flash is very brief and you are unlikely to notice much difference between short and long electronic flash exposures either as you shoot, or in the results.