It was built in 1976 by Canadian National[?] (CN) who wanted to demonstrate the strength of Canadian industry by building a tower taller than any other in the world. The tower was originally designed as a radio and television transmitting antenna. There are microwave receivers for distant signals at about 305 metres, and the top of the transmission antenna is at the apex of the tower.
At 342 m is the Glass Floor and Outdoor Observation Deck, at 346 m is Horizons Cafe and the Indoor Observation Deck, and at 351 m is the 360 Restaurant, which rotates once every 72 minutes. Food is extremely expensive. (Don't plan on just ordering drinks and an appetizer -- read the fine print before you get on the elevator.)
The Sky Pod (a tiny little enclosed platform elevated high above the main observation floor) is located at 447 m, and is the highest public observation deck in the world. Admission to the Sky Pod is extra.
See also: World's tallest structures