Archer Queen is stronger than Archer, of course. Archer Queen is unlocked once you hit Town Hall 9. She will be summoned automatically once you build the Archer Queen Altar with 40,000 Dark Elixir. Like Barbarian King, she is immortal. After every battle, she need to sleep to regenerate her HP. You can’t bring her into the battle (except Clan War) and she can’t protect the village If she is sleeping. You can’t use her in Clan War or normal raids If she is being upgraded. Like buildings, you can boost her for 2 hours by using Gems. If your Queen reaches level 5, she will have a special ability called Royal Cloak. By activating this skill, she will become invisible to the enemies for a while, her damage will be increased, her HP will be restored and she will summon up to 12 Archers. That’s why I always recommend you get level 5 Archer Queen as soon as possible.
After every 5-levels, the Royal Cloak ability will be upgraded to the next level. Its maximum level is 8.