The example demonstrates the filtering capabilities of Telerik RadGrid. Filtering can be enabled/disabled using RadGrid.AllowFilteringByColumn or GridTableView.AllowFilteringByColumn properties. Then each column that supports filtering (GridBoundColumn, GridCheckBoxColumn, etc) shows a filter box beneath the corresponding header.
User can set a filter expression based on the rules specified in the corresponding column properties: FilterFormatString, CurrentFilterFunction, CurrentFilterValue . After pressing the filter button (next to the filter box) the grid displays only the records matching the filter criteria (regarding the settings of all filter boxes).
Additionally, if you set AutoPostBackOnFilter property of the corresponding column to true you can trigger postback filter operation for Contains (for string types) and EqualTo (for numeric types) when you type in the filter box and press [Enter] from the keyboard. Moreover, you can execute other type of filter action by setting the CurrentFilterFunction property of the corresponding column to an internally recognized value, for example:

< rad:GridBoundColumn DataField="ProductName" HeaderText="ProductName" SortExpression="ProductName" UniqueName="ProductName" CurrentFilterFunction="StartsWith" AutoPostBackOnFilter="true" >

The filtering menu of Telerik RadGrid is presented by single object server-side. This way of implementation has been chosen to speed up the grid performance by merely creating one menu instance server side and cloning the instance for different columns. The filtering menu is independent for each column in RadGrid - this means that the filtering menu options vary by the DataType of the corresponding column. Hence integer column will have one set of filter menu options (EqualTo, NotEqualTo, GreaterThan, LessThan, etc.), string column will have additional options (Contains, StartsWith. etc.) and so on.

Note that the built-in filtering feature is not supported with simple data-binding (calling DataBind()). See the Simple data binding demo from the Populating with data section for more info about the limitations of this binding mode.

Further information about grid known filter functions and filter list options you can find in these articles from the grid documentation:
Filter list options
Grid known filter functions
Filtering chapter

Additionally, totals are displayed in the Quantity column footer (the sum is calculated in the ItemDataBound handler in the code-behind). Further information about how to display totals in the grid footers you can find in this section from the grid documentation:
Totals in grid footers