Active stretching involves contracting the opposing muscle (antagonist) to the muscle being stretched. This does not involve use of props or aids for the stretch like a strap or even your body weight. Because the stretch comes from your own controlled muscle contraction, this is considered a quite safe way to stretch. Imagine lying on straight on your back and lifting one leg as high as you can until you feel restriction in the hamstrings. By engaging the quadriceps (the antagonist to the hamstrings) or body will use reciprocal inhibition to allow the hamstring group to relax. Note that the contraction of opposing muscle is the only force stretching the hamstrings.
This type of stretching is done while completely relaxed with the use of some type of outside help. This outside help can come from a device or product, use of gravity, or a partner. Again imagine lying on your back with one leg lifted until you feel resistance from the hamstrings. If you then hold you foot with your hand or a strap and use your arm strength to pull (and hopefully gently pull) your leg deeper into the stretch, this is considered a passive stretch. A partner could also gently push your leg deeper into the stretch. Because the stretch force is coming from an outside source, the chances of injury are significantly higher in passive stretching than that of active stretching.
This is the most common type of stretching. In static stretching, the muscle, or muscle group, is elongated to its farthest end-feel and then held for a certain length of time. Static stretching can be done both passively or actively.
Ballistic stretching involves bouncing a muscle in short and quick bursts at or near the muscles end range of motion. Imagine folding over to touch your toes and then bouncing up and down at the bottom of the stretch. This form of stretching used to be promoted as a pre-exercise way to prevent injury. More recently, ballistic stretching is considered to be dangerous if not performed appropriately.
As its name suggests, dynamic stretching is a more functional stretching technique that moves through a muscles full range of motion in an exaggerated manner. It is often used by athletes. The body’s momentum carries the muscle past its normal end range of motion to encourage a stretch. This should be done methodically and slowly to avoid the muscle’s stretch reflex. Imagine standing up and gently swinging your leg forward and backward as high as you are able to go. This is the idea of dynamic stretching as a preparation for running, for example. Its important not to confuse dynamic stretching with ballistic stretching.
This is similar to static stretching but rather than simply holding the stretch, the stretched muscle is contracted isometrically for a brief period of time and then relaxed. When the muscle is stretched close to its end range of motion and then contracted, the golgi tendon organ, via the reflex arch, will stop the muscle from creating excessive force against resistance as a protective mechanism against injury. Isometric contraction also helps to strengthen the weak parts of the muscle, which over time helps the brain to feel safer in a deeper stretch. Isometric stretching an a very popular and relatively safe way to gain more flexibility relatively quickly.
PNF, or Proprioceptive Neuromuscular Facilitation, is a very popular technique that involves passive stretching and isometric stretching in a particular way to gain maximum flexibility. Note that it is not really a type of stretching but rather a technique for stretching.