CityVille by Zynga is built off its success with FarmVille, FrontierVille, and CafeWorld. As a building simulation game, it can be used as a tool to teach decision-making and simple economics -- which trade route or crop brings you the best yield, or which business is the most profitable if franchised, for example. However, the game can be very addictive as it provides very simple action/reaction type rewards. Interaction with another city can reward the player with coins, experience points, reputation points, energy, and resources. Energy is the most important item in the game as most click actions require energy, including building anything, collecting rents, and even harvesting crops.
You gain energy over time and can earn it by visiting neighbors, having it be gifted, or buying it (with real money). In the early stages of the game, where a player might have only 10 buildings and farm plots to interact with, this is hardly an issue, but as the player's city grows, the need for energy becomes more urgent. Players will either have to create a large circle of neighbors or purchase energy in order to advance in the game -- the former can create privacy problems if players "friend" strangers.