February '08 and newer releases
There are two databases your report can use: the operational database and the reporting database. The reporting database is a copy which is constantly being updated from the operational database. Occasionally, the copying process can fall behind and updates will not show immediately.
You should use the operational database for reports such as "My Inbox" that run on small amounts of data but need to have the very latest data. For large reports that run on historic data, such as a report that analyzes incident creation rates over the last 6 months, you should use the reporting database. The reporting database allows you to run larger queries, but may not have the last few updates made today. Reports using the reporting database will not allow you to use the "Fill Inbox" button, since the data that they are using is potentially old data.
To determine which database your report is using, edit the report, and on the Home tab select the Options button and then More Options. The Edit Settings dialog will open. Near the bottom there is a label "Data Source" and you can choose either Report Database or Operational Database.
In November '08 and newer releases, new tracking functionality has been added to reports in regards to the particular databases. They now keep track of whether or not a person has set the database used in the report. If a user has not specified which database the report will use and a user subsequently tries to run the report on more data than is allowed by the operating database, then the report will automatically switch to using the reporting database. If the report is edited, it will revert back to using the operational database. To stop this automatic switching from occurring, edit the report and the specify a database. This will mark the database as "user set" and the report will no longer switch.
For more information on changing report options, refer to the RightNow online help section for this topic.