In a general periodical index, you'll find articles from magazines/journals or newspapers that deal with a wide range of topics...written primarily for the non-specialist. Many general indexes will search for both *popular magazine articles and newspapers and scholarly (or peer-reviewed) journal articles.
Examples:
Readers' Guide to Periodical Literature
Academic Search Premier(EBSCO)
Expanded Academic Index(InfoTrac)
Wilson Select Plus
Newsbank (searches only for newspaper articles!)
Subject indexes will lead you to articles geared to a more specific topic and written for a particular audience (typically researchers). These indexes will search for articles from scholarly journals only.
Examples:
Education Index (an education periodical index)
Medline (a medical periodical index)
General Bussinessfile (a business periodical index)
Agricola (an agricultural periodical index)
Steen library subscribes to over 150 electronic databases!
*What's all this about "popular magazines" and "scholarly journals"?