TTL stands for Through-the-Lens. Different types of TTL exist:
E-TTL (Evaluative Through-The-Lens Mode)
E-TTL flash exposure control is a flash mode that reads through the lens, but not off the film. Utilizing a preflash fired after the shutter button has been fully depressed but before the camera's reflex mirror goes up, E-TTL uses the camera's evaluative metering sensor to analyse and compare ambient light exposure values with illumination reflected from the subject by the preflash. This data is used to calculate and store the flash output required for optimum exposure of the main subject (identified by the active focusing point), while maintaining a subtle balance between foreground and background. Though transparent to the user, E-TTL represents Canon's most advanced flash exposure control system to date. E-TTL is used in EX-series Speedlites.
A-TTL (Advanced Through-the-Lens) Mode:
The predecessor to E-TTL, A-TTL is used in EZ-series Speedlites when set for direct flash in the camera's program mode. (TTL flash exposure control is available at other times.) Like E-TTL, A-TTL reads through the lens and concentrates its sensitivity on the area of the picture covered by the active focusing point. Unlike E-TTL, A-TTL controls flash exposure with a dedicated sensor that reads flash illumination reflected from the surface of the film during exposure. A-TTL also uses a preflash, but unlike E-TTL, the preflash is used to calculate an aperture value based on the distance that light must travel from the flash to the subject. The camera compares the distance-based aperture data to the ambient-based aperture data calculated by the camera's metering system, and selects the smaller aperture value of the two. This method ensures accurate exposure of the subject in any lighting condition. Speedlites equipped with A-TTL are fully compatible with all analogue EOS cameras.
TTL (Through-the-Lens) Mode:
TTL is the automatic flash exposure control mode used by the small pop-up flash units built into some EOS cameras. It is also available with EX-series Speedlites when used with non-E-TTL EOS cameras*. Additionally, TTL is the standard flash mode for Macro Ring Lite ML-3, Speedlite 200E and Speedlite 480EG. TTL is identical to A-TTL in almost every way, except there is no preflash. It can be used in every camera exposure mode. In Program mode, TTL always selects an aperture value based on the level of available light.
*EOS 620/650, 750/850, 600, 1, RT, 10, 700, 1000/1000F, 100, 1000N/1000FN, 5, 500, 5000, 1N, 1N RS and 3000.