Difference in Chain Types What is the difference in chain types, some say 11 speed, some say 9.

Is the only difference length, what would happen if i put an 11 speed chain on my 9 speed cassette ?

thanks.
The amount of room on the rear wheel hub for the sprocket cassette is limited. In order to get more sprockets into the cassette, they have to be closer together. To 
accommodate this chains have to be made narrower.  

The internal width (the width of the rollers between the inner plates) is the same for different speed chains - 3⁄32" (2.4 mm). The external width is less for higher speed chains.

It's my understanding that using a 1-speed higher chain is generally workable, but 2-speeds higher will definitely cause shifting problems.

The spacing of chainrings on the cranks also has to be narrower for higher speed chains.  I believe there is the possibility for a too-narrow chain to drop in between the chainrings if used with lower speed cranksets.