"ondemand": Reveals the description when the user mouses over the slideshow, then hides it again when the mouse moves out.
"always": Shows a persistent description panel at the foot of the slideshow that can be dismissed by clicking on the "x" icon.
"peekaboo": (v2.4 feature). Reveals the description for each image automatically by sliding it in view each time, then hiding it again before the next image is shown. This setting is similar to "always" above, except the description isn't always visible, allowing viewers to see each image in its entirety momentarily.
"none": Explicitly disables the descriptions from showing. While not defining a description for any of your images inside the initialization code has a similar effect, this option is useful when you wish to define descriptions, but just not show them using one of the methods above, but rather, in combo with the onslide() event handler for example.