It is a certification that gives products and services a green rating.
Coming directly from the Green Seal web site:
"Founded in 1989, Green Seal provides science-based environmental certification standards that are credible, transparent, and essential in an increasingly educated and competitive marketplace. Our industry knowledge and standards help manufacturers, purchasers, and end users alike make responsible choices that positively impact business behavior and improve quality of life.
A 501 (c)(3) nonprofit organization, Green Seal issued its first environmental standards in 1991-2, and the first product certifications were completed in 1992. Hundreds of products and services from major companies such as 3M, Benjamin Moore, and Andersen Windows have now been certified to meet Green Seal standards, and the number of major product categories covered by standards has increased to more than 40."
Getting a GreenSeal means that a product or service has met the Green Seal standard for a particular product line. Such as a Benjamin Moore line of paints meeting the Green Seal standard for "Paints and Coatings".
This certification is industry recognized and means more than just "Being Green". It means that you are doing more than just changing to all CFL light bulbs and calling your hotel green. It means your product has met multiple green qualifications to get the Green Seal. And that makes Green Seal the industry standard.