A standard DVD has a single layer 4.7 GB disc, commercial DVD has more than one layer.
"In DVD recording, a standard, single layer, 4.7 GB disc holds up to 2 hours of video at DVD quality. All commercial movie DVDs hold about 5GB per layer - which each layer holding about 133 min. If you buy a movie DVD that has a 2 hour movie, plus an hour or so of extra features, this means that the disc has more than one layer. Some DVDs are also dual-sided.
All currently available DVD players and recorders can play back commercial discs with more than one layer. However, some older players (pre-1999) may not be able to in all cases. "