It's two stories that intertwine. If you pick Mint in the beginning, you play as a spoiled princess who seeks a wish-granting relic to usurp her older sister and take over the world. Pick Rue, you'll play as a mysterious boy who wants to use the relic to bring back his dead friend Claire.
The characters play fairly different. As Mint, you'll use two large rings for (weak) physical attacks and more powerful magic projectiles. As Rue, you'll use an axe and his ability to shapeshift into monsters you've defeated. As both characters, you'll have to use your unique skills to solve puzzles and traverse 3D environments.
It's a really great action-RPG, and possibly my favorite game on the PSX. I played it again recently on the Vita, and I really feel like it's held up. The combat may be fairly simple, but the dialogue, characters, music, and art make it worth playing today.