QGIS/resources/context_help/QgsAttributeTableDialog-en_US

<h3>Attribute table</h3>
The attribute table displays features of a selected layer. Each row in the table represents one map
feature with its attributes shown in several columns. The features in the table can be searched,
selected, moved or even edited. 
By default the attribute table is a separate window. If you opened it and cannot see it anymore, it is most likely hidden under the main QGIS window. You can make it also a docked window, by checking <label>Open attribute table in a dock window</label> in <label>Settings > Options > General</label>.<p>

The number of table features is printed on its header.<p>
<p>
<a href="#Selecting">Selecting</a><br/>
<a href="#Sorting">Sorting</a><br/>
<a href="#Searching">Searching</a><br/>
<a href="#Field">Field Calculator</a><br/>

<a name="Selecting">
<h4>Selecting</h4>
</a>
Rows can be selected by clicking on the row number on the left side of the row. Selecting a row
doesn't change the current cursor position. Multiple rows can be marked by holding the
<label>Ctrl</label> key.<br>
A continuous selection can be made by holding the <label>Shift</label> key and clicking on several row headers
on the left side of the rows. All rows between the current cursor position and the clicked row are
selected.

<a name="Sorting">
<h4>Sorting</h4>
</a>
Each column can be sorted by clicking on its column header. A small arrow indicates the sort order
(downward pointing means descending values from the top row down, upward pointing means
ascending values from the top row down).<br>
Use <label>Move selected to top</label> button to sort selected rows and display them in first table rows.
To show selected records only, use the check box <label>Show selected records only</label>.

<a name="Searching">
<h4>Searching</h4>
</a>
For a simple search by attributes on only one column the <label>Look for</label> field can be used. Select
the field (column) from which the search should be performed from the drop down menu and hit the
<label>Search</label> button.<br>
For more complex searches use the <label>Advanced search</label>. This button
opens the Query Builder and allows you to select table features matching a
SQL-like WHERE clause.

<a name="Field">
<h4>Field Calculator</h4>
</a>
The <label>Field Calculator</label> button in the attribute table allows to perform calculations on basis of
existing attribute values or defined functions, e.g. to calculate length or area of geometry features.<br>
The results can be written to a new attribute column or it can be used to update values in an already
existing column.<br>
You have to bring the vector layer in editing mode, before you can click on the <label>Field calculator</label> icon to
open the dialog.