DVDs are for more than playing movies. A DVD can play all kinds of mediafrom videos to slideshows to musicbut it's also a big storage device. A DVD holds 4.7 GB of dataactually more like 4.3 GB (the discrepancy is partly because of the way computers measure the capacity). A double-layered DVD holds almost twice as much, though you need a newer SuperDrive capable of burning these larger-capacity discs. Regardless of capacity, there are primarily two types of DVDs. One kind (a regular DVD) holds movies, and the other (sometimes referred to as a DVD-ROM) holds software and files.
iDVD has the great capability to combine these attributes, even burning double-layered versions if you have the hardware. You can play movies from your disc, and you can also store files in folders that recipients access by popping the disc into a Combo drive or SuperDrive (or any other DVD player hooked to your Mac). In the latter case, the DVD would be just like a giant CD-ROM.
At first, there were only CD players in Macs. Then there were drives that could both play and record CDs. Then came DVD players. Since the DVD format is backward compatible, as they say, a DVD player can also play a CD. Next was a drive that could play the DVD, but both play and record a CD. This is a great set of features, and Apple sells this as a Combo drive. Finally, there was the DVD player and recorder, which also plays and records CDs. This is a SuperDrive.
The video and other media content on a DVD must be encoded in order to play on your DVD player and TV. The files are squeezed (compressed) to fit on the disc, which decreases their quality slightly but makes them considerably smaller. The digital video files you work with in iMovie are large (1 GB per 5 minutes of video, as you may recall), but after iMovie prepares them for the DVD, the same gigabyte can hold between 15 and 30 minutes of video (depending on some of your preferences in iDVD, but don't worry about that now). Consequently, a single-sided DVD can hold up to 2 hours of your videos.
This compression is one reason you can't go backward, moving videos from DVDs into iMovie, without a lot of work.
Because of the way iDVD squeezes video to fit on the disc, recording less material to the disc (say, an hour or less) will yield slightly better quality than putting on more. For the best quality on single-sided discs, keep the length of your DVD material under an hour.
When you use a DVD for data storage (as a DVD-ROM), you don't want to compress files at allyou want to leave them exactly as they are. iDVD has the capability to manage all of this, which is one of the excellent features of the product.
There are many, often confusing, DVD formats, which you may have come across once you got a SuperDrive. There are DVD-R, DVD-RW, DVD+RW, double-sided, dual-layered, and more. While most are generally interchangeable and play in a Combo drive as well as typical consumer DVD players, some won't. Never mind the subtleties between formats; the blank discs available from Apple tend to record easily in SuperDrives and play without problems on most players. And from here on we'll talk about DVD-R single-sided discs.