Blank DVDs come in two different formats: write-once and rewritable. Write-once DVDs are called DVD-R. This means that they are only for one-time usage. The data that you burn onto the DVD cannot be erased or replaced with other information. A rewritable DVD, or a DVD-RW, can be erased and used over and over again.

Plus and Minus Format

Write-once and rewritable DVDs can be classified as two types: plus (+) and minus (-) formats. The plus and the minus formats correlate to the DVD burning drive. Some drives may only support the plus format, while some only support the minus format. Fortunately, many drives support both.

Storage Space

A single-sided DVD+RW disc can store up to 4.7 GB (gigabytes) of content, while a double-sided DVD+RW disc can store up to 9.4 GB of content.

Rewriting Durability

Unlike the DVD-R, which cannot be used more than once, the DVD-RW disc can be rewritten about a thousand times.

Speed Ratings

Both write-once and rewritable discs have different speed ratings. They can be two times the standard write speed (2x), eight times the standard speed (8x), and so on. This number corresponds to the speed rating of the drive, or how fast it will burn the disc. For the best results when burning a disc, it's always best to match the speed rating of your blank DVD to your drive.

Amount of Time to Burn a Disc

The amount of time it takes to burn a DVD really depends on the speed of the drive and the amount of information being transferred to the disc. The length of the video may not necessarily determine the length of time it takes to burn the disc; for example, a half hour of video at high data rates can take more space on the disc than an hour at low data rates.

Source:

Windows XP Magazine

DVD+Rewritable: How It Works

DVD Demystified

More Information:

DVD FAQ