Read an online ruler the same way as a physical one, except move the object to the screen, rather than the ruler to the object. Used accurately, both rulers suffice.
Online rulers are more accurate but less convenient than physical ones. An online ruler website recognizes your monitor's resolution and adjusts the size of the ruler image on the screen with a high degree of accuracy. Physical rulers, typically made out of wood or plastic, wear over time and become inaccurate. Though not as precise, many prefer physical rulers because they are easy to transport and do not require electricity for measurements.