Passion is a strong emotion, usually related to love or anger. If on your first time out together your date falls on their knees and tells you they'd die for you — they're gripped by passion (or maybe just nuts).
We associate passion with any intense feeling, but centuries ago it referred specifically to intense pain. The sufferings of religions martyrs, who were tortured and killed for their beliefs, were called "passions," from the Latin passio, or suffering. Today we've dropped the torture, and most of us, when we're not in the grip of passion, have a passion (or intense interest) — for things like gardening or golf or dollhouse architecture.