DUAL LAYER, SINGLE LAYER DVD's
I am by no means a guru, so I'll take an easy one
Single Layer - DVD-5
When DVD's were introduced in early 1997, they came in a format called a DVD-5 (which industry jargon for a disc that is has the movie on a single side, written on a single layer). This meant that a little over 2 hours could be fit onto the disc. The surface of the disc itself was silver.
Another version of this was the DVD-10: it had two sides, each side containing a single layer. The DVD-10 was basically two DVD-5's glued together, back to back.
Simply put, all of the information contained on the disc fits onto one "layer."
Dual Layer - DVD-9
Towards the end of 1997, gold-surfaced discs were manufactured. These were dual layer (DVD-9), meaning twice as much information can be held on the same side of a disc, split over two layers.
The disc is literally 2 seperate layers, glued on top of one another. The laser reads the first layers, and then refocuses and can read the second layer
through the first layer!
Initially production was very low, although now most discs are DVD-9.
THE LAYER CHANGE
During viewing of a dual layered disc, a slight pause might occur at some point during a film. Sometimes it is more obvious than others.
The cause is simple: the laser reading the disc reaches the end of the first layer and is required to shift to the next layer to continue. The laser refocuses and can read the 2nd data layer thorugh the first one (and is even able to read through the thin layer of glue that holds the two layers together!)-- but it takes a split second to get on track.
Some films have good, almost unnoticeable layer shifts, some actually lose a bit of dialogue or music during the shift.
Other Types - DVD-14 and DVD-18
Terminator 2:UE (the single disc version) and Superman: The Movie were both DVD-18. This means they are two sided, each side containing dual layers: for a total of 4 layers glued together! Because of the thin layers, these were harder to manufacture, but allow immense storage capacity on a single disc.
Recently, The Terminator and Legally Blonde have been released as what's called DVD-14's (dual layer on one side, single layer on the other- total of 3 layers glued together).
Advantages of dual layer: Disc capacity allows greater bit budget (i.e. more data) to be used without having to switch or flip discs. This allows dts soundtracks, multiple commentaries, better average video rate (which gives a better picture) or longer movies to fit on a single disc (or side).
Disadvantages of dual layer: The layer shift. Also, some people have encountered dual layered discs that seem to have the glue layer breaking down (called "dvd rot"). No one is sure what causes DVD rot, or if it even really exists- but some early dual layer discs have become unplayable over time.
Disadvantages of two sided disc: Flipping the disc to utilize certain content. Also, some complain that with double sided discs, there is twice as much surface to risk scratching (and single sided discs usually have nice silkscreened artwork).
Overview of Jargon
Single layer, single side -> DVD-5
Dual layer, single side -> DVD-9
Single layer, two sides -> DVD-10
Dual layer on one side/
Single layer on the other -> DVD-14
Dual layer, two sides -> DVD-18