There are two methods of attaching a rope to your body: you can tie the rope round your waist using a figure of eight knot or a bowline; alternatively you can use a harness. The advantages of using a harness are that if you have to hang on the rope the load will be spread across your body. So you are less likely to be injured in a fall and the harness will be more comfortable to hang in than a rope tied round your waist. The disadvantage of a harness is that it is more kit to buy and more to carry on the mountain.
Modern harnesses are designed to keep the user in the upright position when hanging on the rope, and they come in a variety of designs. The two main ones are those with leg loops or those with a nappy design.
Leg-looped harnesses are more awkward to use in winter, as you either have to step through the leg loops or fiddle with three different buckles with cold fingers. A nappy design, like the Troll Alpinist, has only one load-bearing buckle, and as a result it is lighter and easier to put on.
In winter you will be wearing more clothes than normal so you need to make sure to allow for adjustment. The extra clothing means that you don't need a padded harness; this will save weight. A couple of gear loops will be useful when climbing.
The practical uses for a harness when walking are fairly limited. Most of the uses discussed in the rope section of the site can be achieved with the rope tied around the waist. However, if you are planning to move onto steeper terrain, or easy climbs, a harness is well worth the investment.
With all specialised pieces of mountaineering equipment, do get some thorough training before you need to use the gear.