Glaciers don't only look extreme, they feel extreme. Hiking along on a glacier on a sunny day you can become hot and sunburned but a few hours later you may encounter a blizzard or wind, causing temperatures to plummet. If you fall into a crevasse, the temperature will be frigid. Using a layering system that ventilates easily is extremely helpful in this environment. Wear moisture-wicking base layers that will act as a first layer of insulation (polypropylene, polyester or merino wool) and midlayers that act as your main form of insulation (down, fleece, or wool). A good midlayer is a long-sleeved fleece under a down jacket. Have outer layers with you that are windproof and waterproof. A jacket shell and pants that have ventilation zippers at the sides are preferable for minimizing sweat.
Other essentials are hats, balaclavas, bandannas, neck gaiters, glove liners, gloves or mittens, several changes of socks, and a helmet. Polarized sunglasses and sunscreen are a must. Snow blindness and bad sunburns happen quickly because of the reflection from the white surface of ice and snow.
Always inspect your gear before you embark on a glacier travel trip. Make sure that it is sound and that there are no signs of weakness in any used gear. Make sure your buddies are checking their gear as well. Glacier travel requires that your group bring a dynamic rope of at least 8.5 to 9 millimeter thickness but no more. A rope of this thickness will hold crevasse falls and is several pounds lighter than a standard 11 millimeter rope. (Standard ropes are 165 feet long.) If you plan on doing vertical climbing in addition to trekking, use a rope of at least 10 millimeters thick. Also if possible use a "dry" rope (ones that have a water repellent treatment and absorb less water in the snow). If you don't use a dry rope it will be fine, but your rope will get a lot heavier to carry.
Each person in the team should have a rescue pulley, anchor, two single-lengths and one double-length runner, a belay device such as an ATC and carabiners at least one of which should be a locking carabiner. These are all used for rescue pulley systems. A shovel is also good to have among the team for flattening out a place to camp or for digging anchors and wands are useful for markers and identification points.
You will need a basic climbing harness that will fit over several layers of clothing, a mountaineering ax to help with balance and in case of self arrest or boot-belays. Crampons are of course a necessity; these allow you to maintain traction on the ice even at steep angles. Crampons with horizontal front points are fine for glacier travel and ones with ABS to prevent snow from balling up on the bottom in wet conditions are helpful. Prusik slings must always be carried on your person at all times. These are used as ascenders to climb a rescue rope should you happen to fall into a crevasse and need rescuing. Crampon compatible mountaineering boots (leather or plastic) should be worn. Boots with removable liners are nice because you can put the liners in your sleeping bag at night to prevent them from freezing. Make sure your boots work well with the skis or snowshoes you may be bringing on the trip. Your sleeping bag should be a rated for 0 degrees or less. Many people bring two sleeping pads when sleeping on a glacier to insulate better from the cold. Your tent should be a 4-season tent.