Your boots need to defend against water in all of its different iterations so that your feet stay dry and toasty. This can be achieved through many different ways, but the focus should first be on construction, materials, and design details.
The best construction methods are those that are highly water resistant or waterproof - like Goodyear welt construction or cement construction - because they will prevent water penetration at the base of your boots. By contrast, construction methods like Blake/Rapid make for good summer shoes, but are not suited for fighting off moisture because water will seep through the stitching.
Materials also impact water resistance, since you want something that will keep water out, while still being breathable enough for sweat to evaporate so that your feet stay dry. A high quality oiled leather works incredibly well among natural materials to keep water out (and can be further enhanced with the application of natural protectants ranging from beeswax to linseed oil). Gore-Tex is the gold standard for synthetic fibers and creates a more technical look.
Though it may seem obvious, boots with design details like higher shafts allow you to safely walk through deeper snowbanks and puddles without water coming in over the top. While it’s possible to take this to extremes, any shaft 6” or higher will cover you in most normal situations. A gusseted tongue can marginally deflect water from reaching your foot through the tongue, but in reality, this enhancement is pretty minor. It also comes with the disadvantage of making the boots hotter and harder to put on, meaning it is more of a personal preference than a definitive improvement.
Water resistance is created through the thoughtful combination of the right construction, materials and design details. Done properly, water resistance means your boots will withstand whatever winter throws your way, while keeping your feet dry and comfortable.