In addition to those items you bring on a day hike, http://colorado-wilderness.blogspot.com/2009/12/your-day-pack-contents-what-to-carry-on.html, your backpack will contain those things necessary for spending one or more nights in the wilderness. Because you'll be carrying everything for miles, sometimes with significant elevation gains, it's important to consider the weight and necessity of each piece of camping gear you put in your backpack.
Tent - lightweight backpacking tents have been around for a while; depending on the season and climate of your trip, you may go with as little as a rain fly or as much as a four-season tent
Sleeping bag - possibly the bulkiest thing in your pack, the right sleeping bag can make a big difference in your comfort
Sleeping pad - inflatable foam core sleeping pads insulate you from the cold ground and give you a soft surface on which to place your sleeping bag
Biodegradable soap, camp towel, and other toiletries
Toilet paper and trowel (plus the knowledge of how to use them in the wilderness)
Lightweight stove and fuel, pot for boiling water, cup or mug, spoons, water filter
Second flashlight - I prefer LED flashlights, and will take one handheld and one headlamp
Garbage bag (pack it in - pack it out)
CLOTHING - in addition to what you would pack or wear for a day-hike, bring extra clothing depending on your comfort level wearing dirty clothes; a fleece vest might be the only additional insulating layer you'll need
FOOD - some people take great pride in their ability to make "gourmet trail meals;" I prefer to think food-is-fuel, and bring things that are dense with calories, low in water (weight), and easy to clean up; see (insert post url here) for a complete discussion of trail food
to be continued...
