MOSS Search Query Syntax - Keyword, SQL and URL
This blog posting applies to Microsoft Office SharePoint Server 2007.
MOSS 2007's search engine supports three different types of query syntax: Keyword, SQL and URL.
MSDN article Building Windows SharePoint Services Search Queries gives a reference for these syntaxes. What it doesn’t make clear is that MOSS's out-of-box Search UI only exposes Keyword Syntax, and that custom development is needed to utilize the other syntaxes.
On a related note, you can use SQL Syntax for the ContentQuery web part by specifying your query in CAML. MSDN article How to: Customize the Content Query Web Part by using Custom Properties discusses this.