Update - Clarified the ability to play without friends.
The newest iteration of SimCity adds online features, letting you connect your friends' cities with ease and create an inter-working economy. Being a sim city, Maxis is attempting to create an approximation of how a real economy works. Just like cities can have friendly relationships and rivalries, your SimCity can too.
An online connection is required, but you don't have to play with anyone. You could set up your own private world with a system of interconnected cities if you'd like. But it will be easier to use the built-in friends list. The CityLog gives updates on what's happening around the other towns, and Maxis will regularly make global events that will require people to work together to complete large goals.
Perhaps one of the most interesting additions to the game's online functionality is the global market, which allows the buying and selling of commodities. Some resources are finite, and prices will fluctuate based on demand. You'll only be able to trade within your region.
"When you get real personalities involved, compared to playing by yourself or AI, the game takes on an extra depth to it," lead designer Stone Librande told IGN. "Sure, your neighbor might be a jerk or he might not play the way you really want to, but in terms of feeling like you're running a city and dealing with problems real cities have to face? It's kind of a cool story comes out of it."