Global warming (at a basic level) is caused by gases being trapped inside the atmosphere that act as an incubator. There are many different ways that these gases can be released into the atmosphere and trapped there; for example, volcanic eruptions release lots of gases that get trapped. However, the emissions of gases from most factories, cars,trains, airplanes and power plants release over 10 times as much as the volcanoes. For more information, you can check out this interactive website (show your teacher, too!):
global warming.

Global warming, and climate change in general, is an ill-understood phenomenon and we do not really understand its nature or its cause. Since the early Twentieth century (as long as we have been measuring it reliably), the Earth has been warming up, glaciers have been receding, and so on. One possible explanation is that volcanic activity of the Twentieth century is less than that of the Nineteenth (think Krakatoa, Tambora, etc.), leaving less dust in the air. Also possible (although unlikely) is that the sun is becoming brighter, or that there are other shifts in weather patterns or ocean circulation directly at work.

Part of the concern about global warming is that it may be in part caused by the increased amounts of carbon dioxide in the atmosphere resultant from the burning of fossil fuels by humans. There are some difficulties withthis hypothesis; however, as carbon dioxide is still a very minor component of the Earth's atmosphere and more importantly it is not a very potent greenhouse gas (water vapor is much more effective). Because of the possibility that human pollution may have something to do with it, global warming has become a political issue, which is never a good thing for a science: there are now special interest groups fudging data on either side, and so it is difficult to know whose data to even trust, let alone make decisions based off of.