Why are there A and B tags for general elk hunts, and what's the difference?

The simple answer is that generally A tags favor archery and muzzleloader hunts, and B tags favor centerfire rifle hunts. The A-B zone tag system was established as an effort to offer elk hunters the most general season choices. An example: an archery hunter in central Idaho's Sawtooth Zone, with an A tag, can hunt any elk in the four units of the zone during September. If unsuccessful, the archer still has the opportunity to hunt spike elk during a short any-weapon season in October. (There is also a short muzzleloader A tag season on antlerless elk in November.) With a B tag, the early archery season would be about two weeks shorter and for spikes and cows only. But the later any-weapon season for a bull would be longer, running from mid-October into November. This example doesn't hold for all elk management zones, and some have no general hunts for elk. In the end, hunters should decide what they want to hunt and where and find the hunts that suit them, be that an A tag or B tag.