A Flow City is usually community built by several players all buying tiles within close proximity to each other. By far the flow cities currently in E2 are the most impressive in terms of tiles sold.
The main advantage to this city type is the shear scale and numbers of people involved to make it work.
The risk in this city type is that depending on where you buy your lot, you could be "built in" by other players and unable to easily access the rest of the city. However, many assume that players will be able to walk on player owned land and walk around structures to reach desired destinations. These flow cities may likely create natural pathways that assist in the "flow" of traffic within the city.
Most Developed and Structured cities will have flow type suburbs in the outskirts.
Flow City example (Eye of the Sahara)