Search Relationships define how Criteria Row are processed together. The default search relationship is the And relationship, but Or is also available.
In the following example, you can see the And and Or relationships used, along with indenting to define grouping between the Criteria Rows.
The logic actually used by the above search: (Date Accessed >= '9/22/2004' OR Date Modified >= '9/22/2004') AND (ItemID <> 1000). See the section on Indenting for the details.