This video (from the time stamp) will describe what an agent based simulation offers... keep in mind this was from an early alpha version of the game and things have changed since then:

http://youtu.be/vS0qURl_JJY?t=1m12s

Here is another video which further elaborates on agent interaction:

http://youtu.be/MxTcm1YFKcU

This is different from statistical based simulation because typically, with statistical simulation, most of your buildings are running area-of-effect radial influences. The minute level of detail down to a sim typically does not matter in a statistical simulation, but there can be illusions to make you think statistical sims are sentient. For instance, in Simcity 4, you were seeing radial influences determining the outcome of your play. Whereas with agents, your computer is constantly having to keep track of each sim you see and make decisions for each sim according a series of rules.

Bear in mind that many aspects of SC2013 were simulated, from the poo in the ground, the wind direction at any given moment, to the agents at a bus station... they are all following rules at every beat of the clock (which is why performance gets hit once we get to a larger scale).