How to reset your Mac's NVRAM, PRAM, and SMC
Zap the NVRAM (or PRAM)
Back in the day, the standard list of quick fixes for random Mac ailments always included "zap the PRAM." The P in PRAM stood for parameter (the RAM was just RAM--random access memory), and it referred to a small amount of special, battery-backed memory in every Mac that stored information the computer needed before it loaded the operating system. If the values in this memory got out of whack for one reason or another, your Mac might not start up correctly, or might exhibit any of numerous odd behaviors afterward. So you could press a key sequence at startup to reset (or "zap") the PRAM, returning it to default, factory values.
Modern Macs no longer use PRAM; they instead use something called NVRAM (NV for non-volatile). NVRAM serves approximately the same purpose as PRAM, but instead of storing a dozen or more pieces of information, it now contains just a few: your selected startup disk, speaker volume, screen resolution, time zone, and--if your Mac has crashed recently--details of the last kernel panic.
NVRAM corruption is fairly uncommon, but if your Mac seems to take forever to figure out which disk to boot from, if it starts up with the wrong screen resolution, or if you have weird audio problems (like there's no sound whatsoever, or the menu bar's volume control is grayed out), it doesn't hurt to reset the NVRAM--it's quick and harmless.
To reset your NVRAM, you use exactly the same procedure you once used to reset PRAM: Shut down your Mac, press the power button, and as soon as you hear the startup chime, hold down Command-Option-P-R. Keep holding down those keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally. Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure they're set the way you want them.
If you hold down Command-Option-P-R at startup and you see nothing but a gray screen that doesn't change for several minutes--no Apple logo, no progress bar, no second startup chime--don't panic. (This happened to me just last week.) The most likely cause is that your Mac isn't registering the key presses due to wonkiness with a USB device. Disconnect all USB devices (except your keyboard, if it's a wired keyboard), hold the power button down until the Mac shuts off completely, and then press it again and immediately hold down Command-Option-P-R. If that doesn't work and you're using an external Bluetooth keyboard, try plugging in a USB keyboard instead. If you're able to reset the NVRAM successfully with the wired keyboard, you can disconnect it and go back to your normal Bluetooth keyboard.
Reset the SMC
Yet another component of your Mac's hardware that stores crucial settings is the System Management Controller (SMC), a circuit that deals with power management, temperature monitoring and fan control, status lights, keyboard backlights, and a few other components. If your SMC becomes confused, you could experience problems like excessive fan noise, slow performance even though Activity Monitor doesn't show the CPU being overtaxed, apps that take forever to launch, batteries that don't charge correctly, problems with sleep or wake, and so on. (You can see a longer list on this Apple support page.) As with zapping the NVRAM, resetting the SMC to factory defaults may resolve these problems.
Apple says you shouldn't reset the SMC without first trying other troubleshooting tasks, such as force-quitting problematic apps and restarting your Mac. However, Apple doesn't mention any negative consequences of resetting the SMC, nor any way to determine for sure if the SMC is glitchy without resetting it and noticing that the problem went away. I've reset my Macs' SMCs many times with no apparent ill effects, and on occasion that did in fact turn out to be the solution to a problem.
Before you can reset your SMC, you must shut down your Mac. After that, the procedure varies depending on the type of Mac you have.
Desktop Macs: Disconnect the power cord (either from the Mac or from the AC outlet). Wait 15 seconds and plug it back in. Then wait another 5 seconds and turn the Mac back on.
Portable Macs with non-removable batteries: Make sure the Mac is plugged in to AC power. On the built-in keyboard, press and hold the Shift, Option, and Control keys on the left side and press the power button. Release all the keys at once, and then turn the Mac on normally.
Portable Macs with removable batteries: Disconnect the AC power cord and remove the battery. Press the power button, hold it for 5 seconds, and then let go. Put the battery back in, reconnect the power cord, and turn the Mac on normally.
Although neither of these procedures is a guaranteed cure, both of them can solve a number of odd problems, and are worth a few minutes of your time before hauling your Mac in to see the local Apple Genius.