Rulers

The ruler is extremely important as it displays all your tabs and indentation settings for the active textbox.
The ruler is also useful when you are working with other objects and can help you to align objects correctly.
You can display the ruler by selecting (View > Ruler). This is a toggle.
This can help you to position your objects as well as displaying tabs and paragraph indents.
It is possible to display both a horizontal and vertical ruler in the Slide Pane of the Normal view and the Notes Page view.

Horizontal Ruler

When there is nothing selected the numbering on the ruler starts from the centre of the slide. This is to help you position objects.

Vertical Ruler

When you are positioning objects on a slide you might also find it useful to display a vertical ruler as well.
The vertical ruler can be displayed by changing your (Tools > Options)(View tab, "Vertical ruler").

If the have the vertical ruler displayed then the (View > Ruler) checkbox will toggle the display of both the horizontal and vertical rulers.
When a textbox is selected the numbering starts from the top left corner of the textbox.

Different Measurements

It is possible to change the unit of measurement displayed on the ruler from Inches to Centimetres.
Inches

Centimetres

This cannot be changed from within PowerPoint but it can to changed using your Windows Control Panel.
Select (Start > Control Panel), Select Regional and Language Options.
The precise options are different for different versions of Windows but there should be an option to change the Measurement system between US and Metric.
Choosing Metric will display the measurements in Centimetres and choosing US will display the measurements in Inches.

Important

If you are using PowerPoint 97 the ratio from centimetres to inches is incorrect. PowerPoint uses 2.4 cm per inch when the accepted standard is 2.54 cm. For more information please refer to the Microsoft Support Article 189826.
If you are using PowerPoint for Mac, versions 2001, X or 2004 you can change the measurement unit from (Tools > Preferences)(View tab, "Ruler Units").