The first incarnation of DVD held 1.46GB on a single layered disc (referred to as DVD-1) Over time the spec improved all the way up to the current 4.7GB (now referred to as DVD-5). Dual layered discs are an improvement of DVD-5 technology by allowing 8.54GB of space which has been achieved by the addition of a transparent layer of data (this is known as DVD-9). If you want to get crazy and who wouldn't, there are also a few DVD formats above DVD-9 which are basically double sided discs: 2 single layers (DVD-10), 1 single and 1 dual layer (DVD-14), or 2 dual layer (DVD-19).
Blu-ray also supports dual layers, there's a 25GB single layer disc version and a 50GB dual layer version.
HD DVD was supposed to offer a third layer but they lose.
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