Difference between Refernces of Engine Libs and Desktop Libs?

Hi all.
I've in the middle of writing a program and i don't know -when adding ArcGIS references- which of Engine and Desktop libraries i've added. In my computer. i've both ArcGIS Desktop and Engine installed, so in Add ArcGIS Reference dialog i see libraries for both which have the same name. for example, ESRI.ArcGIS.SpatialAnalyst is in both Desktop Extention category and Engine Extension category. Is there any difference between them, regarding license in deployment phase (i.e. for clients)?