Phytoremediation is the use of plants to clean pollutants out of water, soil, air, sediment. It falls under the more general term of bioremediation, which includes other living things that can be used to clean pollutants, for example fungi or different types of bacteria. Just think of the prefix bio as life and phyto as specific to plant life, combined with the word remediation. On the down side the remediation generally goes only as far as a plant’s root system can reach, we are not sure if the pollutants are transferred to bugs and animals that eat the plants, and there is concern that the pollutants can still leach out into the groundwater. Some typical pollutants are metals, sewage, explosives, arsenic…
Using the example of a contaminated industrial site where toxic metals have sept into the soil, phytoremediation is a lot more effective and cost efficient than removing contaminated soil or sealing it in concrete. It looks a lot nicer too.. Constructed wetlands are one method of phytoremediation, 

the EPA published an informational document called “The Citizen’s Guide to Phytoremediation”. It is very basic, easy enough elementary school students to understand. http://www.clu-in.org/download/citizens/citphyto.pd
another EPA guide is more advanced with a lot of short articles starting on pg 27 of the pdf ww.epa.gov/tio/download/remed/phytoresgude.pdf
Phytoremediation is being used at Fresh Kills on Staten Island for the dual purpose of cleaning the soil and building up a layer of organic material to cover the ground. (I think you need a NY Times log in for this one.) http://www.nytimes.com/2005/02/24/garden/24natu.html?pagewanted=2&8bl
army is using phytoremediation to clean explosives from soil: http://el.erdc.usace.army.mil/resbrief/phytoexp.html
Plants have been used in Chernobyl clean up: http://www.mhhe.com/biosci/pae/botany/botany_map/articles/article_10.html
And, it is cleaning the former steelworks plant, now a park, Landschaftspark Duisburg-Nord that I wrote about in an earlier post.
One newer and interesting aspect of phytoremediation is phytomining. Research has shown that some plants that extract metals can then in turn have the metals extracted out of them. Below is an excerpt from: Phytoremediation: Using Plants To Clean Up Soils”
Published in the June 2000 issue of Agricultural Research magazine.
In 1998, ARS agronomist Rufus L. Chaney and colleagues in ARS, at the University of Maryland, and in England patented a method to use such plants to “phyto-mine” nickel, cobalt, and other metals.
Chaney says biomining is the use of plants to mine valuable heavy-metal minerals from contaminated or mineralized soils, as opposed to decontaminating soils.
“The crops would be grown as hay. The plants would be cut and baled after they’d taken in enough minerals,” Chaney says. “Then they’d be burned and the ash sold as ore. Ashes of alpine pennycress grown on a high-zinc soil in Pennsylvania yielded 30 to 40 percent zinc—which is as high as high-grade ore. Electricity generated by the burning could partially offset biomining costs.”
full article at http://www.ars.usda.gov/is/pr/2000/000622.ht
I don’t know the implications of burning all this plant material, but it is amazing that we have found a way to recover metallic waste.