Bluetooth, define descriptors for characteristics ?

Well, the only thing I know is, in a sample there is this:

• - Descriptor: Characteristic User Description
  
• - Descriptor: Client Characteristic Configuration
  
• - Descriptor: Characteristic Presentation Format
  Found some more information in O'REILLY Getting started with Bluetooth Low Energy.
  To be honest, no. of questionmarks in my face is more than before.
  I'll try to ignore them for now :-)