From Todd Bleeker ...

The out-of-the-box SharePoint WSS Search performs a full text search through all of the list items and document libraries (including attachments) in the current WSS site. This search box is found in the upper right corner of a WSS SharePoint web page. Searches via the WSS Search box are limited to the current site. Sub-sites cannot be searched without first going into the sub-site.

The WSS Cross-Site Search Web Part performs the same type of search, except it searches through sub-sites as well. In addition, the starting URL for the search can be customized. As a result, a WSS Cross-Site Search web part placed on one site can search for results within another.

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