The Johnson Space Center is NASA's center for human spaceflight activities, aka "Mission Control."
The cool (and surprising) thing is that most of the modern space programs and technologies are housed and operated in the original NASA buildings, as opposed to some sleek new facilities down the road. For example, today's mission control room is just one floor below the original control room where they put a man on the moon. I guess the people that work here are pretty smart. Not only did they put a man on the moon (allegedly) but they also know exactly how many steps are on each flight of stairs. Pretty impressive.
Houston is also where NASA does the Space Shuttle, Skylab, Lunar Rover, and neutral buoyancy training. We saw all that, some massive liquid hydrogen tanks, and the original Saturn V rocket, which is one big mofo.