The TTL flash are the latest in flash. What is TTL? Means'' Through the Lens'' or'' through the lens.''
The process of calculating the required amount of light gets through the lens unlike the Auto mode is done with a photocell on the flash.
TTL mode functions differently in the chambers analogies (SRL) and the digital (DSLR).
In analog cameras when the light hits the film part of this light is reflected to a sensor that controls the flash.
When this sensor believes there is enough light to expose the photo correctly, short flash.
In digital cameras this is not possible because the CCD or CMOS sensor does not reflect too much light. So how does it work? The flash will fire pre-flashes before exposure.
It measures the light coming through the lens and used to estimate exposure. This happens so fast that the human eye perceives it. We only see a flash shot.
The first flashes were Canon E-TTL and the improved system is E-TTL II. In the first Nikon D-TTL system was and the improved system is i-TTL.
In the pictures below you can see a flash of Canon TTL and Nikon i-TTL.