DVD-5 is an acronym that means a DVD disc that is one-sided single-layer disc and can contain upto 4.38 gigabytes of data on it.
DVD disc manufacturers love to use marketing terms and sell their discs as 4.7GB discs, but this is not true -- the 4.7GB is calculated by using so-called "Japanese gigabytes", where the power of calculations is 1,000 instead of 1,024 (and 1.024 is the correct way to calculate everything in computer world -- so, 1024 megabytes == 1 gigabyte).
DVD-5 DVD-Video discs are nice, because DVD writers (this applies to both, DVD-R and DVD+R standards) can only write single layer discs.
Other regular DVD size acronyms are:
-DVD-9
-DVD-10
-DVD-14
-DVD-18
