Plain gauge: same diameter along the whole length of the spoke
DB: Double butted, thinner in the middle than the ends
DT Competition: a type of DB spoke, 2.0mm at the ends, 1.8mm in the middle.
DT Revolution: a lighter, and thus flimsier, double butted spoke, 2mm at the ends, 1.5mm in the middle, or occasionally 1.8/1.5.
Also get bladed like DT Aerolite or Sapim CX-Ray, which are flattened, giving higher fatigue resistance and better aerodynamics, whilst costing far more.
Basically… plain gauge are the cheapest and heaviest, often agreed to be weaker than DB.
DT Comps are an excellent all round spoke for any wheel, Revs are good if weight matters.