Dry, comfortable, socks which keep your feet from blistering rates up there with a warm, dry, sleeping bag, as far as essential gear.
Good insulating hiking socks are one of your most essential pieces of gear for backpacking.
The better socks are wool/synthetic (polyester) blends, of aproximately 90% wool. Wool blends are more comfortable than 100% wool, and don't shrink as much or take as long to dry as straight wool.
Socks with varying degrees of insulation can be purchased. It goes without saying you need more insulation in cold weather backpacking. Some hiking socks also have extra padding at the heel and the ball of the foot, where blisters are more likely to develop.
What you don't want to do is go backpacking with cotton socks. Cotton is quickly wetted by foot moisture, and dries slowly. It provides no insulation when wet, and is a good way to get blisters and sore feet.
One of the best things you can do is to bring along extra socks. I usually have about four extra pairs with me on my backpack trips.
On my outing last weekend I changed my socks twice a day, because I was walking around all day in melting snow.
I came back with sore feet though, because I should have stopped and switched socks again during the long hike out. I didn't take the time, and for that I paid the price. Fortunately I didn't get any blisters however.
Rei has a very good article on hiking socks, in their expert advice area.
Hiking socks are not cheap. If you don't need them right away, you might search froogle with the term 'hiking socks' or 'wool hiking socks', and then sort by price from low to high. Given enough time you might come across some good bargains.
