What was once 401 North Michigan Avenue, The Hub is the centerpiece of the city; the city's tallest and most prominent building. There are 100 floors. This is where the Choosing Ceremony takes place, where children that are of the appropriate age choose what faction they will join and delegate themselves to for the rest of their lives. Peter Hayes implies that Abnegation, as the faction that control the government, have control in The Hub. It is assumed that the Hub serves as the formal seat of government in future Chicago much like a city hall. The Hub also seems to hold meetings for the city council with the representatives of each faction.
Relations to Present-Day
In real life, The Hub could be located in many different towers in Chicago. For example, 401 North Michigan is what is presently the Trump Tower, which is only the 4th-tallest building in Chicago, with 98 floors. The actual tallest tower in Chicago in present day is the Sears Tower, at 108 floors. It is located at 233 South Wacker Drive. Is is also possible, but highly unlikely, that The Hub could be a building constructed before the events of Divergent, but after present day. This is unlikely because the Trump Tower is 98 levels high, and it would be nonsensical to to build a tower only 2 floors higher. In addition, the shape of the Trump Tower's site does not permit for a building to be any shape but a triangle. Therefore, if a new tower was constructed at 401 N. Michigan, it would be almost identical to the Trump Tower.