AAC, MP3, DRM-free, bitrate -- the amount of the techie terminology you're required to know before you can get involved in the world of music downloading can be enough to drive people right back to the CD stacks. Here's what you need to know:

All About iTunes
Apple's iTunes website remains the most popular hub for music downloaders -- it's a place where you can buy, store, organize and play your music all in one place. With iTunes' online store, you can purchase individual songs for $.99 each or digital "albums" for less than the cost of traditional CDs. Once you purchase one of these song files from iTunes, it is automatically added to your iTunes library. Your library is the online equivalent of that bookshelf in your room where you store all your CDs. If you'd like, you can even import your entire physical CD collection into your iTunes library - this way you can store all of your music (CDs and downloads) together in one place.

iTunes also operates as a music player which can play your downloads, as well as any music from CDs you may have loaded, directly on your computer. If you don't want to listen to your music on your computer, purchase some blank CDs and use iTunes' "Burn" feature to copy (or "burn") your music onto CDs, which you can then play on your regular CD player. You can burn a whole album or make a custom "mix" CD. Any song can be burned an unlimited number of times, but iTunes limits you to 7 copies of any album or specific custom playlist.

iTunes is also designed to work seamlessly with Apple's iPod portable player and its iPhone cell phones. Songs from your iTunes music library can be easily transferred to your iPod or iPhone, allowing you to listen to your music collection anywhere. If you don't want to listen to your iPod with the ear buds, you can purchase a docking station, which is essentially a speaker system that connects to your iPod. Adapters are also available which allow you to play your iPod on your car's speakers.

The Upside?
One of the most obvious advantages of buying downloads is the "a la carte" feature, which allows you to purchase individual songs. No longer are you required to buy an entire CD just so you can listen to that one song on it that you like. iTunes also allows you to be your own DJ - you can create your own playlists and play songs in whatever order you decide. Portability is also a great plus. You can load up your iPod with your entire music collection and take it with you anywhere. Finally, CDs take up space. With downloads, your entire music collection can be stored on your computer.

The Downside?
When you download music, it's easy to run into all kinds of compatibility issues. Different online stores sell different formats of music downloads. Napster sells WMA downloads, a format developed by Microsoft. All of the songs sold by iTunes are coded in AAC, a format developed by Apple, while eMusic, Zune and Amazon.com sell MP3s. The problem with this multitude of formats is that not all formats can be read by all digital players. For example, iTunes and iPods will only play MP3s and AACs, not WMAs. If you want to avoid compatibility issues, buy MP3s. The universal format of MP3s will play on virtually every digital player.

Another good reason for buying MP3s is the fact that they are DRM-free. DRM (Digital Rights Management) is a form of copy protection. Companies that use DRM embed a code in your download that restricts what you can do with it. For example, when you purchase a download from Napster, DRM prevents you from playing or storing that song on more than three computers. iTunes also uses DRM. WHile an individual song purchased on iTunes can be burned to CD an unlimited number of times, DRM prevents you from making more than 7 copies of any album or specific custom playlist. iTunes' DRM also prevents you from storing and playing that song on more than five computers and from playing it on any portable player other than an iPod or iPhone.

MP3s don't have any such restrictions. When you purchase an MP3 song, you can make an unlimited number of copies of it, store and play it on an unlimited number of computers and burn it to an unlimited number of CDs. iTunes does sell a selection of DRM-free songs -- these are designated as "iTunes Plus" songs -- but they are coded in AAC, a format that is not universally recognized by all digital players, so you may run into some compatibility issues. For example, the Windows Media Player (this is the digital player that came pre-loaded on your PC) doesn't recognize iTunes Plus songs. iTunes also embeds your iTunes user name and email address in the code of your iTunes Plus download, so it remains traceable.

Sound Quality
The difference in bitrate between CDs and music downloads is one reason why some audiophiles still prefer to buy CDs. The bitrate, measured in kilobits per second (kbps), refers to the amount of digital information that is transferred per second. So, as a very general rule, the higher the bitrate, the better the sound quality. CDs have a bitrate of 1411 kbps. For music downloads, the standard bitrate is usually anywhere from 128 kbps to 256 kbps, which is less than one fifth of the information contained on a CD. Music downloads are purposely designed to be small, compact files -- otherwise they'd take up too much room on your computer or iPod. But to create these small files, over 80% of the data contained in the song is thrown away - this includes sounds that are beyond the range of the human ear as well as the virtually imperceptible background sounds that are drowned out by more dominant sounds. The average listener who plays their music on an iPod or computer probably won't be able to tell the difference between a CD and a music download, but audiophiles and those with sensitive, high-end audio equipment might.

In addition to bitrate, sound quality is also determined by which program (or "codec") was used to create your music download. Each codec uses a different set of rules to determine what data is thrown out of a song. The most popular formats include AAC (all iTunes music is AAC), MP3 and WMA. Many people feel that AAC is superior to MP3 in terms of sound quality, such that a 128 kbps AAC song sounds as good as a 160 kbps MP3 song.

Music Subscriptions
Music subscriptions provide you with an entirely new way to listen to music. With a subscription service, you pay a monthly fee (usually around $5-$10). In return, you get access to the site's entire music catalog (usually 1-3 million songs).

You can play these songs on your computer as often as you wish, but you cannot burn these songs to a CD.

For an additional fee (usually a couple of dollars more per month), you can also download all the songs you want to a portable music player. Compatible players include the Rhapsody Sansa and the Creative Zen Vision:M, but if you're an iPod user, you're out of luck. Subscription services won't work with an iPod.

Here's the catch...if you cancel your subscription, all the songs you downloaded to your computer and/or portable music player will disappear and will no longer play. If there are songs you want to keep forever (even if you cancel your subscription), you can buy them for around $.79-$.99 each. Unlike the subscription-accessed songs, purchased songs can be burned to CDs.

You'll also need to make sure you sync your portable device with your service every month, or the licenses on your songs will expire and you'll end up with a bunch of unplayable music on your MP3 player.