The Association of Authors' Representatives (if you thought it was the "Association of Authors Representatives" or "Association of
Author's Representatives" you might want to reconsider your literary ambitions, my friend) was formed in 1991 through the merger of the
Society of Authors' Representatives (founded in 1928) and the Independent Literary Agents Association (founded in 1977). Basically, it's
the professional association for literary agents which, among many other things, maintains the Code of Ethics that you should expect any
literary agent to follow, whether or not they're a member of the AAR. One of the most important things about the AAR's Code of Ethics is
that it forbids members to charge "reading fees" (which, if you're looking for a literary agent, you might be asked for).
But lack of membership in the AAR should not necessarily be an onus for an agent. In fact, many high-profile agents
(e.g., Binky Urban) are not listed on the AAR's website, for the
simple reason that they don't need to be listed on any more websites and get inundated with any more submissions (except through
Bill's List, of course ;-)).