Defining your industry in basic terms means deciding who your competition is. There are several ways to go about this:
1. The US government defines industries by NAICS codes (they used to do this with SIC codes). Go to http://www.census.gov/eos/www/naics/ and type your (one word, if possible) industry description in the search box on the left side of the page.
2. Reverse Engineer. If you know a company that belongs in your industry, use the Hoover's Online database and search for your company. Once on the company page, scroll down the middle of the page until you find the "Industry Information" box. Note the NAICS code(s) for your company.
3. Ask me. I help students with this portion of their research all the time, especially for difficult to define industries.