Many people use the term 'passive' and 'static' stretching interchangeably, though they are not quite the same. Static stretching is when you stretch to the farthest point and hold the stretch, while passive stretching, while also being a static stretch, is a technique in which you are relaxed and make no contribution to the range of motion. Instead, an external force is created by an outside force such as a partner.
Passive stretching is also referred to as relaxed stretching, and as static-passive stretching. Another static stretch is the active stretch, where you actively hold the muscles in a stretched position.
A passive or static stretch is one where you gradually ease into the stretch position and hold it with some other part of your body, or with the assistance of a partner or some other apparatus. For example, bringing your leg up high and then holding it there with your hand.
The splits is an example of a passive stretch (in this case the floor is the "apparatus" that you use to maintain your extended position). The amount of time a static stretch is held may be anything from 6 seconds to 2 minutes. Often in static stretching you are advised to move further into the stretch position as the stretch sensation subsides.
Slow, static stretching is useful in relieving spasms in muscles that are healing after an injury. Obviously, you should check with your doctor first to see if it is okay to attempt to stretch the injured muscles. Static stretching is also very good for "cooling down" after a workout and helps reduce post-workout muscle fatigue, and soreness.