A search index is a more general classification than a browse node. For example, one search index is Books; a browse node within that search index might be Harry Potter Books.

As you can imagine, there can be many browse nodes within each search index. Some Product Advertising API search operations require a search index to limit the scope of the search. Specifying a browse node in addition to a search index returns more targeted search results. For example, a search operation, ItemSearch, that looked in the Books search index for book titles and descriptions that contained the word "dragon," would return thousands of titles. The same ItemSearch request with the Harry Potter browse node specified would return only those Harry Potter books that had dragons in them.