Plains (along with Grassland) are one of the most widespread terrains in the world. It is much more balanced than Grassland because its soil, although less fertile, is also rich in minerals and provides a balanced yield of the two most important stats for cities: Food and Production. That makes it the second most suitable terrain to settle, because ensures that a city may produce things much more quickly right from its onset. Like Grassland, they are also abundant in terrain features and resources.
Plains trades off population growth for increased production. A city built on plains will have good Production, especially if it has any Hills and Woods nearby, but growth will be slow until Farms are established. In this aspect it is good to look for Food-enhancing features and Resources, which will help population growth early on.
The presence of Rainforests can help growth considerably, mostly due to the resources found there. The extra Food point for flatland tiles will make the difference between a normally growing and slowly growing city; still, you should look at every opportunity to build growth-enhancing buildings, such as a Granary.
A plain is simply a large, flat bit of terrain, with soil not as rich as fertile grassland but not as dry as desert. Thus, left alone (before all those tractors show up) plains tend to be overgrown with dry grasses and grains ... perfect for large herds of animals such as bison, antelope, horses, and barbarians.