For the VAST MAJORITY of bicycles, spokes will be in a range of 1.5, 1.6, 1.7?, 1.8, 2.0 & 2.3mm.
Single butted spokes will have a larger diameter at the J bend.
Double butted spokes will have a larger diameter at both ends.
Sometimes the term triple butted is used if the J bend, center and threaded end are all different diameters from each other.
Such as-
http://www.dtswiss.com/Components/Spokes/DT-alpine-III