The benefits of the web far outweigh the risk of inappropriate use. As educators we should strive to train our students on acceptable web behavior including the habit of summarizing, quoting, and citing sources. Despite our best efforts, some students choose to bend/break the rules of fair use.
It is important for educators to be aware of the tools and resources that are available to monitor and deter cheating in the classroom. Turnitin.com is a tool that can be used to check written work for plagiarism. It is a very sophisticated and effective web resources that can quickly scan and find portions of a paper/essay that have been copied from the web or another paper. The downside to turnitin.com is its price, which is prohibitive for many individual teachers and many schools. The service also requires a bit of legwork on the part of teachers and students.
A free, simple, alternative that I have been using for several years is "The Plagiarism Checker" from dustball.com. Paste in a bunch of text from a suspect paper and The Plagiarism Checker will quickly perform a Google search of multiple portions of the submitted text in an effort to match
The basic service is free to use. A more advanced and accurate checker is available for $8/month.
I have personally used this tool on student work. Some students were vindicated while others were busted.