Turning to the Shadow
Turning is a complex process, requiring thirteen channelers to weave flows of the Power through thirteen Myrddraal. To the Aes Sedai's knowledge, Turning has not been done since the Trolloc Wars (TDR, Ch. 22). However, it was done routinely at the Black Tower under the direction of Mazrim Taim before Tarmon Gai'don (A Memory of Light, Prologue).
A person who is Turned does not act as though they are under a state of Compulsion. Instead, their darker elements are elevated and their more positive altruistic elements suppressed. Not everything about a Turned person changes. They keep their memories, their personality, and their failings (AMoL, Ch. 14). Lanfear believed that a person who had been turned lacked the "spark of ingenuity" (AMoL, Ch. 6).
It is unlikely that any such person could find a way back to the Light, partially because they would have no desire to do so (Robert Jordan Question of the Week, Week 15). After a person is Turned their eyes look cold (ToM, Ch. 53), glassy (Towers of Midnight, Epilogue), or like they have a darkness behind them (AMoL, Ch. 3, AMoL, Ch. 4). When they smile it looks completely unnatural, like a grimace or the smile on the lips of a corpse (ToM, Ch. 53). Some people believe that the victim is killed when Turned and something else invades the body (AMoL, Ch. 6).
The more willpower a person has, longer it takes and the harder it is to Turn them (AMoL, Ch. 4).
It is easier for male channelers to Turn female channelers, and for female channelers to Turn male channelers. Toveine is Turned easily because she was Turned by men (AMoL, Ch. 14).
The more dedicated to the Light a person was before being Turned, the more dedicated to the Shadow they become afterwards. The inverse is also true: if a person is not so strongly dedicated to the Light before being Turned, they will not be as strongly dedicated to the Shadow. (AMoL, Ch. 3).
People who were Turned
The following are known to have been turned to the Shadow:
"This is something known only to a few, child, even in the Tower. You should not learn it now, if ever, but I will tell you. There is a - weakness in being able to channel. That we learn to open ourselves to the True Source means we can be - opened to other things. ... It is a thing not done, so far as I know - Light send it has not been done! - since the Trolloc Wars. It took thirteen Dreadlords - Darkfriends who could channel - weaving the flows through thirteen Myrddraal. You see? Not easily done. There are no Dreadlords today. This is a secret of the Tower, child. If others knew, we could never convince them they were safe. Only one who can channel can be turned in this way. The weakness of our strength. Everyone else is as safe as a fortress; only their own deeds and will can turn them to the Shadow." (Sheriam, The Dragon Reborn, Chapter 22).
"These had been Turned. You could see it in their eyes - eyes that weren't quite right, weren't quite human." (Isam; A Memory of Light, Prologue)
"Well, that was certainly not as bad as I had assumed it would be, Androl. Don't worry about me. I feel great. No more fear, no more worry. We shouldn't have been fighting all of this time." (Evin to Androl, after being Turned; AMoL, Ch. 8)
"You can stop resisting. Relax, Logain. Give in. ... Let's see this done, once and for all. He deserves the peace of the Great Lord's bounty." (Toveine talking to and about Logain after she was Turned; AMoL, Ch. 14)