As mentioned above, double knitting is a method of making a two-sided reversible fabric. It usually involves two colors and creates reverse images on either side of the fabric.
If you keep the two sides as two solid colors (i.e. no image/design on it) then you will end up with two layers of fabric that can be pulled away from each other, thus allowing you to turn the fabric inside out if one end is left open (like a pillow case).
If you create a design/image in the knitting then the two sides will become interlocked and will not be able to be pulled apart or turned inside out from the end. Called interlocked or Jacquard double knitting.
While double knitting usually uses two colors, it is possible to do it with more.
It is also possible to create non-reversible/two-pattern double knitting. That is double knitting with two different images on the two sides, rather than reverse images.
Or, as with your bookmark, you can creat identical images on each side of the knitting.
The technique to which Suzeeq eluded is referred to as the slip-stitch technique.
Double knitting can also be accomplished through the alternating stitch technique.