I've seen rotary encoders that you can get with and without detents (example). What are detents in the context of rotary encoders? When would you want / need your encoder to include this feature in a design?
Detents are slight "clicks" the encoder will make when you turn it. Think of what it feels like when you scroll your mouse wheel (hopefully mouse wheels you've used have detents :P). They provide some sort of feedback (audible/touch) mechanism for the user to what a discrete step in the encoder is.
More information: Wikipedia - Detent