Anor and Ithil are LOTRO's progression servers that supposedly allow players to experience previous expansions as level cap content. The servers are open only to VIPs and lifers, and originally were loaded with the lvl 50 Shadows of Angmar content. SSG is supposed to release each xpac in order roughly every 4 months; Riders of Rohan has just been released.
They are somewhat like the classic WOW servers, except that the content (loot boxes; scaled instances) and systems (trait trees; virtues;) mirrors the regular servers. They usually release some new cosmetics or titles along with the xpac.