In TTL, a sensor in your camera body measures the light reflected off your film plane and stops the flash pulse when the film has been sufficiently exposed.
In E-TTL (or A-TTL in Nikon terms), the sensor on your flash unit measures the light reflected off your subject and sends this information to your camera body. The camera then takes into account this info and that from its own sensor, computes an average value and uses that exposure. Generally, E-TTL is more accurate.
The above is a very good and in-depth article explaining how the whole Canon Speedlite system works, including differences between E-TTL and TTL.
PS: It's updated to include E-TTL II as well!
In summary, TTL just reads flash output needed based on reflected light off the film plane while E-TTL evaluates the flash output needed based on a pre-flash before the actual exposure. Apparently E-TTL works better because it is not fooled by light bounced off from e.g. highly reflected surfaces.
This is probably why TTL was never supported in DSLRs. There's no film to read off the reflected light.