Wet - or traditional - shaving is simply the idea of putting and holding as much water as possible on the face while shaving. In practice, this is usually accomplished by using a brush and traditional shaving soap or cream. Many wetshavers continue the traditional theme by using a double-edged safety razor or straight razor.
Modern shaving creams and gels from a can use propellants that evaporate after the product is applied to the face. This evaporation tends to leave air pockets next to the hairs, which is exactly the situation to avoid while shaving. A wet face and wet hairs allow the razor to glide smoothly and slice cleanly. In fact, some straight razor shavers only use water to lubricate their shave, but most shavers prefer some sort of soapy lather. The variety of scents and brands is surprising, considering the percentage of wetshavers in the shaving population as a whole.
Below are links to more specific topics on wetshaving.
Equipment - what I use and why.
Preparation - getting your face ready for a great shave
Aftercare - you've shaved, now what?
DE Blade Sharpening - a test of several blade sharpeners.