Skimming, like Traveling is a way of moving quickly from one place to another without crossing the intervening space. Unlike Traveling, it does not require that a person knows the ground they start (ACoS, Ch. 9). The second difference, is that while Traveling only has one end (LoC, Ch. 2), Skimming has two and the channeler must cross a void to get from one to the other, which they open when they arrive at their destination (TSR, Ch. 57; TFoH, Ch. 54). Nevertheless, it is far quicker than normal travel. According to the guide, Skimming was occasionally used by those less adept, the main books indicate it is strength alone that determines whether a person can Travel and there has been no indication that Skimming requires less strength (TWoRJTWoT, Ch. 3).
Inside the gateway, in the void, there is no sensation of heat or cold (TSR, Ch. 57). The method of traveling within the void is up to the individual. When Rand first Skims, he uses steps until he sees Asmodean ahead on a traveling silver platform. When he notices this, the step Rand is on begins to move to catch up (TSR, Ch. 57).
Within the void, concentration is very important. The stone Rand uses begins to fade when his concentration drifts (TSR, Ch. 57). Similarly, when Skimming to Caemlyn, the platform begins to become insubstantial when he makes it larger; he is unsure if this is because he was trying to increase it, or because it started at his limit (TFoH, Ch. 54). Direction for men is not important, as long as you are heading towards the destination, you can go any way (TFoH, Ch. 54). That people stand on when they skim is up to them. Rand tends to use a platform with nothing to hang on to at the edges, which runs the risk of people falling over if they are not careful (TFoH, Ch. 54). Egwene uses as a wooden barge as her platform, with rails for safety. If she falls, she can make another platform, but then has no control over where she comes out (ACoS, Ch. 12). Similarly, when Beonin teaches Elaida how to Skim, she uses a barge (KoD, Ch. 2).
Egwene learned Skimming from Moghedien, before that, it was not even a Talent that the Aes Sedai remembered. and it is very important to the rebel Aes Sedai as they march towards Tar Valon, each night they stop somewhere new and it would take time to learn the area enough to Travel from it. Instead they fence off an area for making Skimming gateways. They Skim to Salidar and Travel back, those who are strong enough (ACoS, Ch. 9).
Men at least, Skim in a similar manner to the way they Travel, by folding reality (TSR, Ch. 57). As with Traveling gateways, Rand has the ability to stop a Skimming gateway from closing (TSR, Ch. 57). For men, they can encounter other people in the void, if they have the same destination, though starting from close by might be important (TSR, Ch. 57). Women however, can't run into another person unless they start at the same place (ACoS, Ch. 9).
The rebels Skim back to Salidar while on the march (ACoS, Ch. 9).