All commercially available CDs are recorded at 16bit/44.1kHz sample rate. Sample rate refers to how many times the analog wave is scanned per second when it's converted to digital information. At 44.1kHz, the audio wave is scanned 44,100 times every second. While that might seem like a lot, it's possible to have a higher sample rate. The highest sample rate available now is 192kHz, which means that the wave is scanned 192,000 times per second! The higher the sample rate, the better the computer can get a look at the incoming audio and the better it can make a representation of the true sound.
Sample rate is analogous to dots per inch on a printer. The more dots per inch, the sharper the printed image. The higher the sample rate, the better quality the audio sounds.
Sixteen-bit refers to the “bit depth,” that is, the amplitude or loudness of a signal. At 16 bits, the incoming audio can have 65,536 levels of amplitude. If the incoming audio falls between one of the bits, the computer will round it up or down to whatever is closest. As a result, you're not getting a perfectly true picture of the sound. Higher bit depths are available to correct this. Twenty-four-bit allows 16,777,216 levels of amplitude, which as you can guess gives you a more accurate signal.
The new thing in home studio recording is improved audio quality via higher sample rates and bit depth. Sixteen-bit/44.1kHz is perfectly acceptable (and all CDs are encoded that way), but most audio programs accept higher sample rates and bit depth. You'll see 24bit/96kHz advertised, which means it's recording at a higher audio quality. That's the good news. The bad news is, not all software is capable of playing back high-rate audio, and neither can all interfaces. In effect, you can use a high-rate audio program and a 16-bit sound card and never reap the benefit. So be forewarned. Fortunately, most of the current audio cards allow 24bit/96kHz recording, and all the major software packages listed in Chapter 6 work at that rate as well. And yes, you can hear a difference.