From WindsportAtlanta.com: Wiki
Wikipedia definition and explanation
"Fetch, often called the fetch length, is a term for the length of water over which a given wind has blown."
- Fetch on Wikipedia
Effect on Waves
"Surface [wind-driven] waves start small, but as they travel more or less downwind, the waves grow in height, become longer and move faster. Although the Great Lakes are large, the fetches they present to the winds ensure that the waves are under-developed (except in light winds) and tend to be steeper. Research has determined the following characteristics of waves on the Great Lakes:
- the closed boundaries effectively eliminate "swell" (long waves propagating from distant storms);
- when the fetch varies substantially about the wind direction, the largest waves tend to diverge from the wind direction towards the long-fetch direction;
- very under-developed waves move faster than fully-developed waves of the same length;
- the longest waves in an undeveloped sea are much steeper than their fully-developed counterparts.
[source for this information: "Thermal Structure and Circulation in the Great Lakes", F. M. Boyce et al, Atmosphere-Oceans, 27 (4) 1989, 607-642]." 
Windsurfers tend to be more sensitive to wind strength than other sailing craft. While we have a fairly wide wind speed range, dipping below our optimal range can mean having difficulty in planing. Since one of the great joys of windy conditions is planing (as your speed jumps from 5 mph to the low-mid 20s or more), finding conditions conducive to steady and stronger winds is important.
A longer fetch (time and distance that the wind can travel over the water before it gets to you) can have several important benefits:
- A longer fetch reduces or eliminates the effect of shoreline obstacles such as trees or buildings. Such upwind obstacles create turbulence and wind shadows. The effect can extend at a distance from ten to fifteen times the height of the obstacle. Considering that a southern trees (pines and hardwoods) can easily reach 100 ft in height, shoreline trees can affect the wind for a distance approaching a quarter of a mile or more. Locations such as the Outer Banks or onshore ocean beaches can have fetches extending from 20 to thousands of miles and tend to have steadier and stronger winds than many inshore locations all other things being equal.
- A longer fetch increases the period (interval between wave peaks) of wind-driven waves and swell. In windy conditions, this creates greater separation from swell to swell making handling and many maneuvers easier. An extreme example of this is the Great Lakes where winds can generate waves in excess of 30-35 feet in storms. The frequency (closeness) of these waves compared to similar height waves in the open ocean makes them more dangerous to boats and ships. As a practical matter, jibing across a swell where the peaks are only 10 ft apart is a bit more challenging then when they are 20 or 30 feet apart.
Sometimes, a long fetch is not ideal:
- During the spring time, when the dome effect may be present, the cold air dome grows in with the size of the body of water making it more difficult for the warm wind above to displace it and blow on the surface.
- Longer fetches create larger wind-driven waves. Extremely short fetches can provide very smooth water even in windy conditions. Water to the lee of a sand bar or similar low obstruction provides better conditions for speed sailing or high speed maneuvers such as jibes.
Local Application and Notable Examples
Lake Lanier's longer fetches as compared to other lakes makes it the first choice for most intermediate and advanced windsurfers in north Georgia. This same consideration is why certain parks, almost all below Browns Bridge, are preferred. Also, the long west fetch at Van Pugh Park means that the swell will tend to be from the west even in a NW wind. This can make for a delightful sail away from the launch in big (4.5 sail) conditions but mean pounding almost head on into some of the waves on the way back.
Some fairly largish lakes such as Lake Oconee, despite their size, have few areas of long fetch making them little used for windsurfing.
On the other hand, Huguenot Park's tidal basin has, in effect, almost no fetch, being completely surrounded by sand. It makes for a delightfully flat place to sail even when winds require a sail smaller than 5.0 sq meters.