What are IFR departure categories?
The categories are actually approach categories that are also used for departures. From the AIM, 5−4−7. Instrument Approach Procedures


  Aircraft approach category means a grouping of aircraft based on a
  speed of VREF, if specified, or if VREF is not specified, 1.3 VSO at
  the maximum certified landing weight. VREF, VSO, and the maximum
  certified landing weight are those values as established for the
  aircraft by the certification authority of the country of registry. A
  pilot must use the minima corresponding to the category determined
  during certification or higher.
  
  
  Category A: Speed less than 91 knots.
  Category B: Speed 91 knots or more but less than 121 knots.
  Category C: Speed 121 knots or more but less than 141 knots.
  Category D: Speed 141 knots or more but less than 166 knots.
  Category E: Speed 166 knots or more.
  


These categories are used to determine visibility and ceiling minimums for an approach and determine the protected area for circling approaches. As you can see in the departure minimums you quoted, they are also used for departures.