Name: The Great Wall of China
Date of construction: 6th century B.C. through 16th century
Built to keep out: Invaders from the north
Status: Tourist attraction and UNESCO World Heritage Site
Little known fact: You actually can't see it from space.
Name: The Green Wall of China
Date of construction: 2002 through ~2050
Built to keep out: The Gobi Desert
Status: Mixed
Little known fact: Prior to the Wall's erection, the Gobi was advancing south at 3 km per year.
Name: The Great Firewall of China
Date of construction: 2002 (and even earlier) to present
Built to keep out: Ideas
Status: Trivial to circumvent but still annoying
Little known fact: Google, Yahoo, AOL, Microsoft, Skype, and MySpace all self-censor their services for use in China.
Great Wall photo by mooney47.