ANSI is the american national standard institute, the rating refers to " Personal Protection - Protective Headwear for Industrial Workers" which sounds to me like a hard hat, just keep in mind that the rating was updated to ANSI Z89.1-1997, so your helmet may be older. The best helmets in my opinion are the UIAA (Union International des Associations de d'Alpinism) rated one's, like Petzl's, Black Diamond's and Edelrid's. the UIAA is the organisation that rates and approves climbing gear for use in europe. http://www.uiaa.ch/index.aspx
. another good resources is Alpine Caving Techniques which has a great gear section with all the explanations of all certifications and testing done to it.