The syntax is fairly simple. Multiple query terms can be combined with the keywords AND or OR, terms can be excluded by prefixing them with a minus, and specific properties can be searched by combining the property name with a comparator and the value.
This syntax can also be used in standard search fields such as the Dolphin one.
Nepomuk AND KDE
search for everything containing the words "Nepomuk" and "KDE" in any of their properties (this includes the content of files).
Nepomuk OR KDE
search for everything containing either the word "Nepomuk" or the word "KDE" in any of their properties.
search for everything containing the exact phrase "Nepomuk KDE" in any of their properties.
search for everything that is tagged with a tag named "Nepomuk".
+hastag:nepomuk AND -hastag:scribo
search for everything that has a tag labeled "nepomuk" but not a tag labeled "scribo".