DVD-5 is a single-sided single-layer DVD that stores up to about 4.7 GB = 4 700 000 000 bytes and that is 4.38 computer GigaBytes where 1 kilobyte is 1024 bytes(4 700 000 000B/1024 = about 4 589 843KB/1024 = about 4485MB/1024 = about 4.38GB) . Video DVD, DVD-R/W and DVD+R/W support this format. Often referred to as "single sided, single layer".
DVD-9 is a single-sided dual-layer DVD which can fit up to 8.5 GB or 7.95 computer GB which many commercial video DVDs are using today (a DVD-9 is basically two pressed plastic DVD-5s pressed together, which are not burned). Video DVD supports this format but DVD-R/W and DVD+R/W do not support this format.