Example: Select 'Population 2000' in the first drop down box in the table 'Select Criteria.' Enter 10,000 and 25,000 in the two text boxes to it's right. In the bottom table, 'Additional Fields', select field(s) to be returned in addition to the 2000 Population and then click the "Submit" button. You will get back a table with all the counties in Texas that had a population of at least 10,000 but no more than 25,000 for the 2000 Census. In addition, there will be fields for the County Name (always included) and for any field you selected in the second table.
For questions about data sources, see Data Sources.