The measure tool measures length of the visible line, width, height and angle shown in the status-bar.
In the tool options there is a check-box Use info window, if checked a window will show itself when you begin measuring providing you with the same details as before. Use info window is disabled by default.
Key Modifiers (defaults)Edit
Shift - starts a new measure from the point the lasts end, without deleting the old one (useful for measuring angles with respect to features in the image).
Ctrl - puts the tool into straight line mode (angles constrained to 15 degree increments), also (click on the endpoint) creates a horizontal guide.
Alt - creates a vertical guide if the endpoint is clicked.
Ctrl + Alt - allows dragging the measured line, also creates both horizontal and vertical guide (by clicking the endpoint).