I taught Environmental Science a number of years ago and I came across all of this then. Some of this material may be a little outdated. I will be working on it this summer to help out a good friend of mine who will be teaching Environmental Science this year. GIS, or Geographic Information Systems, is basically taking data and plotting it on maps. By mapping the data it is often much easier to spot patterns or trends. In education GIS can be a perfect tool for inquiry based labs.
The simplest GIS activity I've ever done with my class was to pass out maps of South East Michigan that I'd gotten from the Secretary of State or the Department of Transportation (I can't remember which). Then in groups of trhee I had students use markers or crayons to trace all th rivers and streams. By highlighting this data students were able to quickly identify the different watersheds in our area. We live in the Rouge River Watershed which covers a large area and funnels down to one very polluted river. Once the watershed was "outlined" in this way it was easy for students to understand why it is still so polluted even given all the programs aimed at changing this.
GIS becomes even more powerful when we bring the power of computers to bear. Below are listed some free programs for analyzing data. The easiest is probably Google Earth, but I highly recommend giving ArcExplorer a try.