"Sport of the Arts" is the slogan of WGI, and I find it to be rather fitting. Winter guards can be from high schools, colleges, youth organizations and drum corps organizations, or completely independent (many guards fit in this category). While there is a large crossover between the DCI and WGI crowds, there are an increasing number of people who choose to do Winterguard only.
Winterguards perform to recorded music (while there may be exceptions, I have not seen any yet). The shows are rooted in dance, pageantry, and equipment work, commonly including ensemble and solo statements with flag, rifle, and sabre. This equipment work (especially on the weapons) can be extremely demanding; in the higher classes, ensemble sabre tosses with 6 or 7 midair rotations are not uncommon; solo tosses are occasionally seen with up to 8 or 9. Props and "floors" (tarps which cover all or part of the basketball court) are commonly used even by small high school guards to contribute to the design and general effect of a show.
The amount of hard work required by all participants to make a show work well is staggering, and most shows are designed, taught, polished, and performed in the space of four or five months. Guards are constantly gaining new recruits and losing the older and more experienced participants to graduation or age-out (WGI age limit is set at 22 (I think?) for all but world-class guards).
There is more to this than I can attempt to describe in a writeup. Much like drum corps, it is a way of life for many, and the result can be incredible to behold.