The most obvious and essential piece of backpacking gear is the pack itself: anything from loose-bodied rucksacks to large-capacity backpacks with external or internal frames. The type of pack you want depends on your favored backpacking style; overnighters don't require massive bags, but for longer excursions in the backcountry you'll want a pack with enough room and heft to accommodate more supplies. Most importantly, when purchasing a pack, make sure it's fitted properly for your body size, as you'll chafe and strain under an incompatibly-sized one.
Modern backpacking tents are lightweight and stow efficiently. Again, consider your recreation habits: If you only camp out in the summer, an airy warm-season tent is fine; autumn, winter and spring backpackers will gravitate toward four-season contraptions.
Sleeping Bag & Pad
Don't skimp on the sleeping bag, which should be rated at cold enough temperatures to comply with the vagaries of weather. Mummy bags are popular for maximum insulation. For two people, zipping together standard sleeping bags can be a comfortable and cozy option. Some people tote sleeping pads, which may be simple foam layers or partly-inflatable rolls.
On shorter trips, some backpackers shun stoves and bring only ready-to-eat items like pita bread, granola and the like. There are plenty of small, light one-burner backpacking stoves on the market; these certainly encourage creativity in one-pot cooking---or stick to the diverse dehydrated options. Backcountry gourmands can make the most of basic ingredients like rice and beans---even incorporating wild-harvested components like huckleberries, mushrooms (properly identified, of course) or trout.
Invest in a pair of trekking poles if you're a regular backpacker. These high-tech walking sticks will take some of the strain off your legs and shoulders, and can be invaluable for traversing rugged and tricky terrain, from fordable streams to deadfall to steep slopes.
Your shoes, boots or hiking sandals are of utmost importance in backpacking. Break in any footwear prior to long-distance hikes, and cater the style to whatever terrain or activity you're aiming for. Sandals or tennis shoes can be effective for stream-hiking, but you'll want more ankle support for cross-country travel.
Never forgo a good topographic map and sighting compass, which are invaluable tools for route-planning. GPS units have become relatively inexpensive and can be used for tracing routes, backtracking, and marking points of interest or landmarks.