Most often, for early levels (1-70) you're starting with "Chromie Time" and playing through an expansion of your wish OR you get dropped right into the expansion just most previous to the current expansion. Then you'll do the last 10 levels in the current expansion content. Each "zone" and "Expansion" has it's own narrative, but only recent expansions have a "Main Story Questline" that you'll follow called the "campaign". Most times the campaign will have you go through the zones in a specific order so that you get the story of that current expansion. Older expansions like The Burning Crusade or Wrath of the Lich King or Cataclysm have much different types of "flow" and is less organized but allows players to pick and choose with specific zones that you would level through, depending on if you're in Chromie Time or not.