The measure tool is located at the bottom of the eyedropper menu in the toolbar.
Information from using the measure tool can be found also on the Info palette.
You can measure in different units of measurement by clicking on the hard to see little icon of the 'T' bar. Units include pixels, inches, centimeters, percent, etc.
This information is used to track the cursor coordinates on the X and Y and the right side is used to determine the width (w) and height (h) of a selection or transform (bounding box). This is important to know when you need to use a preselected size of a selection.
Measurements are made in whatever format you've chosen on the info palette.
Use the measure stick by click dragging and then releasing to a desired point. Appropriate coordinates will be displayed in the info palette.
The upper right quadrant shows the angle and distance of the measurement. Perhaps this lesson is best saved for the upcoming AdvancedPhotoshop.com but it's good for you to know.
More Tutorials: 
