Standard DVDs

• Standard recordable DVDs are available in DVD-R, DVD+R, DVD-RW, DVD-+RW and DVD-RAM. The disks can hold up to 4.7 GB of data, which translates to about two hours of video.
  
  

Dual Layer

• Dual layer discs have a second physical layer on the disc and double the available storage space to 8.54 GB. The dual layer discs are labeled DVD+R DL, and have a second layer of recordable dye. The format was introduced in 2003.
  
  

Dual Layer Drives

• Dual layer drives shine the laser the through semi-transparent first layer to write data to the second layer. There can be a pause in the playback and recording of dual layer discs while the drive accesses the second layer inside the disc. The recording of a DL disc is much slower than that of a single layer disc. The original drives only recorded at 2x; newer DL drives can record up to 16x speed.
  
  

Price

• When first introduced, dual layer drives and media were significantly more expensive than their single layer counterparts. The price of both has come down, though they are still more expensive. Dual layer media is generally two to three times more expensive than single layer media.
  
  

Other Dual Layer Formats

• DVD+RW DL media, a re-writable dual layer media disc, was developed in 2006, but as of yet (2009) has not been released to the market.