Every achievement in the game, who holds it, and who got there first. Ranks stack — a tiered achievement levels up as you climb, and each rank is its own unlock.
2 secret achievements have been claimed. Once someone holds a one-of-one, its condition is revealed — nobody else can ever earn it.
4 secret achievements exist that exactly one player will ever hold. Their conditions are not published — the only way one surfaces is when somebody claims it.