Who is Briseis? Achilles & Agamemnon argue over her in "The Iliad."

The epic covers just a few weeks in the tenth year of the Trojan War & is set in motion by Agamemnon, the Greek commander, demanding that Achilles give up Briseis, his concubine, whom he was awarded in an earlier conflict. Here is Alexander Pope's description from Book I of Achilles handing her over to two heralds sent to retrieve her. This is what the rage of Achilles is all about:

"I know your message; by constraint you came;
Not you, but your imperious lord I blame.
Patroclus, haste, the fair Briseis bring;
Conduct my captive to the haughty king.
But witness, heralds, and proclaim my vow,
Witness to gods above, and men below!
But first, and loudest, to your prince declare
(That lawless tyrant whose commands you bear),
Unmoved as death Achilles shall remain,
Though prostrate Greece shall bleed at every vein.…"