Winter Variation, abbreviated as WV, is a system for orienting the last layer corners during insertion of the final F2L slot. It is generally used in conjunction with methods in which the last layer edges are already oriented before insertion of the final F2L block, such as ZZ or Petrus.
WV may only be used in last slot cases where the final corner-edge pair are already connected in the U-Layer. In the R U R' case Summer Variation can be applied. After using WV, all the pieces will be correctly oriented in the last layer, and the solver must use PLL algorithms to correctly position them.
There are 27 Winter Variation algorithms total, one for each configuration of corner orientations. WV cases are set up so there is a corner-edge pair in the top layer to be placed in the final slot. Recognition of cases is typically done by looking at 3 of the corners in the top layer (the last corner's orientation is always dependent on the other 3). Since the last slot can potentially be in 1 of 4 positions, the solver must readily be able to recognize mirror cases and apply mirror algorithms in order to use WV.
The benefit to learning WV is that the solver uses fewer moves than directly placing the final c/e pair and using standard OCLL algorithms. The average move count for optimal WV algorithms is 8.07. In addition, PLL algorithms are generally well known, since PLL is the last step of the popular Fridrich method.
The original description and rationale for this variation can be seen in the yahoo speedsolvingrubikscube group here.