The different types of green roofs are generally broken down into three categories: extensive, semi-intensive and intensive. Extensive green roofs have 6" of growing medium or less, are usually not accessible to the public, generally have less plant diversity and require less maintenance.
These green roofs most commonly consist of sedum species as the plant material, as sedums are very hardy and can thrive without much soil. Intensive green roofs have more than 6" of growing medium, are usually accessible to the public, can include more diverse plant material, and are higher maintenance. These green roofs are often planted with larger plants, such as grasses, shrubs, and even trees. Semi-intensive roofs are somewhere in the middle of extensive and intensive. A green roof can weigh anywhere from 10-300 lbs/square foot. Basically, as long as the roof structure can hold the weight, you can do pretty much anything you can do in your backyard, on your roof!
The picture below shows a typical build-up of a green roof.
Green roofs have so many environmental, financial, and social benefits that I wish all buildings were required to have these little ecosystems on their roofs. Some of the environmental benefits include stormwater management (reduced quantity of stormwater and improved quality), waste diversion (by use of recycled materials in the green roof system as well as increasing the life of your roof membrane - therefore, less waste in the landfills!), reducing city heat, improving air quality, creating habitat for birds, bees and small animals...Green roofs also improve the energy efficiency of a building, by reducing heat loss in winter, and keeping buildings cooler in the summer. They also block electromagnetic radiation, block noise and prevent fires.
Green roofs are used to create recreational areas, can be used to grow vegetables, or to provide a safe space for beehives or bat houses! As you can probably tell, I could go on and on about how much I love green roofs and why everyone else should feel the same.