If left to its own devices, a vine over time will transform into a tangled, bushy mass of hard wood and not make the effort to produce a lot of fruit. Sometimes you will see vineyards with untrained vines that creep and sprawl over the ground, but normally any vineyard in a commercial capacity will have its vines trained on a trellis system. By pruning and encouraging the vine to grow on a trellis, the potential for fruit yields are maximized through the proper amount of sun exposure and canopy growth. Not to mention, a trellised vine is much easier to access when it comes to harvesting.
The basic, overall idea when it comes to pruning a grape vine is to leave as much one-year old wood as possible and shear everything else off. That's why, if you ever wander through a vineyard in the winter, you'll likely witness a vast sea of trunks and not much else. Have no fear: the vines aren't sick, or dying; they're actually just going through their annual grape-growing cycle.
Although it might not be as sexy as picking fruit to make delicious wine, pruning is still one of the most critical activities when it comes to life in the vineyard. Come springtime, the vines will definitely thank us...and so will you when you tilt back a glass of byproduct from all the hard work.