If you know one thing about East Asian astrology walking in, it's probably your zodiac animal — the 12-year cycle of Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog, and Pig. It's the most exported piece of the whole system, printed on restaurant placemats worldwide. It's also more precisely defined, and does more work, than most people realize.
First, the precision: your zodiac year isn't the calendar year you were born in — it's the Saju year, which turns over at 입춘 (立春, roughly February 4th), not January 1st. Someone born on January 20th, 1994 is still in the 1993 Saju year — a Water Rooster, not a Wood Dog — because 입춘 hadn't arrived yet. This site's calculator handles that boundary automatically; a lot of casual zodiac charts online don't.
Second, the work it does: your zodiac branch is one of the twelve earthly branches used throughout your whole chart, not just the year pillar, and branches interact with each other in three specific, named ways that show up directly in this site's compatibility (궁합) results.
삼합 (samhap, "trine") groups branches four steps apart into threes, each trio sharing an element: Tiger–Horse–Dog (Fire), Monkey–Rat–Dragon (Water), Snake–Rooster–Ox (Metal), Pig–Rabbit–Goat (Wood). Trine partners are read as moving with an easy, compatible rhythm — not identical, but complementary. 육합 (yukhap, "six harmony") pairs branches in six fixed pairs — Rat–Ox, Tiger–Pig, Rabbit–Dog, Dragon–Rooster, Snake–Monkey, Horse–Goat — said to quietly complete each other, filling in gaps without much friction. 충 (chung, "clash") pairs sit exactly opposite each other on the 12-branch wheel — Rat–Horse, Ox–Goat, Tiger–Monkey, Rabbit–Rooster, Dragon–Dog, Snake–Pig — the relationship folklore talks about most: magnetic, a little combustible, the one people ask about first when comparing charts with someone new.
None of these three are a verdict on whether two people should be together — they're a description of one layer of interaction, among several a full traditional reading would weigh. A 충 pairing between two people who communicate well can be the most alive relationship either of them has ever had; a 육합 pairing between two people who never talk can still go nowhere. What the zodiac layer is actually good for is exactly what it says on the label: a starting vocabulary for describing how two rhythms move against each other — not a filter for deciding who's worth getting to know in the first place.