HVAC-Shmaychvac: Why Do I Care?
Unless you live in a bubble (and that would be awesome), you'll probably have to deal with your heat or air conditioning going on the fritz at some point. If that happens, and you're not a heating, ventilation and air conditioning (HVAC) expert, you'll need to speak basic HVAC in order to get your air back online without being taken by the bad guys.
Repair vs. Replacing
HVACs come in a couple options: as a package that heats and cools from a single unit outside, or as a split system in which the AC condensing unit is outside and the heat pump/coil thingy is inside. So if only half of a split unit breaks, you can just replace that part, right? Not so much. Generally, both should be matched to function at peak, so if one breaks, it's usually advisable to replace both. Otherwise, the less efficient component can drag the new one down, and you're headed for a vicious cycle.

Sometimes it's more about efficiency than repair. If your system is unreliable or your bills are sky-high, upgrading could save you money in the long run. Determine your HVAC's efficiency with an Annual Energy Savings Calculator like the one from NATE, aka North American Technician Excellence.
Bigger Is Not Always Better
One of the most common mistakes people make is getting an HVAC unit that's too large. Oversizing wastes energy, money, and can cause humid, clammy-feeling air. To determine the right size for your home, reputable companies and contractors use the "Manual J" load analysis when installing a new system. Manual J is a calculation that takes into account your home's age, number and condition of its windows, insulation, number of floors, square footage, local energy rates and other factors.

Some companies cut corners by simply basing the size of the new HVAC on the size of your existing one. This is a big no-no. The original system could be out of date, or was never properly sized in the first place.
Be Good to the Planet
While it may cost a little more at the outset, investing in an energy-efficient unit can help shrink your bills in the long run. According to North American Technician Excellence (NATE), furnaces and air conditioners with the ENERGY STAR?? seal use up to 20 percent less electricity than older models.

When shopping for any heating or cooling system, compare their efficiency ratings. Look for the terms SEER, AFUE and HSPF. Air conditioners use a Seasonal Energy Efficiency Ratio (SEER), furnaces are measured in Annual Fuel Utilization Efficiency (AFUE), and heat pump units are rated by SEER for cooling and Heating Seasonal Performance Factor (HSPF) for heating.
Dummy-Proof Your Home
Before you pay for or install anything, be sure your ducts are insulated. The best HVAC in the world won't run at its peak or save you money if you've got bad or leaky duct work. Straight or curved ducts also make a difference. Hard turns make for more difficult air flow. Finally, make sure the connections between ducts are sealed well so no air leaks out.
Keep It Cool, Keep It Cozy
Most manufacturers recommend cleaning your heating filters at least monthly and changing them out at least twice a year to keep things in tip-top shape.
It's All Who You Know
As with just about anything, it's almost as important who does the work as what's being done. Find a solid HVAC contractor.
Can I Get a Bid and References?
Any HVAC contractor worth their salt will provide you with a written bid or estimate detailing the work to be done, equipment installations and price, including labor. Check references, and to be safe, get proposals from three different contractors before making a final call.

Resources: U.S. Department of Energy, Air Conditioning Contractors of America (ACCA), American Council for an Energy-Efficient Economy (ACEEE)