The aim of stretching is to physically lengthen a muscle i.e. increasing the distance between the muscle's origin and insertion. When muscles have more length, they tend to have less tension, in other words they are less tight. Part of the stretching debate is whether stretching does actually physically improve our range of motion, or if the muscle has what we call an increased stretch tolerance, meaning it's just got used to stretching and can tolerate it a little bit more.
Types of stretching
There are several types of stretching, the most common being static stretching, which is the type most of us know as maintaining a position where a strong stretch of the muscle is felt and repeated, this can be done by yourself or with someone stretching you muscle for you. There is also active and ballistic stretching - active is taking the muscle through its full range of motion several times and ballistic is getting towards the end range of motion i.e. when there's a strong stretch - and effectively bouncing in and out of the end range - this style of stretching is not advisable, as it has been shown to put you at increased rick of injury. The last style of stretching is the sort of thing your PT or manual therapist may do with you, where you go through a series of contracting and relaxing a muscle repeatedly.
Stretching before I exercise - do I need to do it?
Performance: Static stretching as part of a warm-up for exercise has been shown to be detrimental to muscle strength / performance in the exercise you do immediately afterwards but so far, this has only been found to be the case in running and jumping activities. The current recommendations are that for sports requiring these explosive activites, dynamic stretching is better. However, if you do a sport where flexibility is required e.g. dance or yoga, then static stretching is shown to be beneficial before your workout.
Injury: Stretching in general has been shown to decrease your risk of getting a musculotendinous injury (based on a 2012 review), that is for example a hamstring tear close to the tendon or an achilles tear, but for muscle injuries in general? The jury's out and as it stands right now, more evidence is needed.
Clear as mud right? So what does this all mean? Well, what appears to be best is actually an individual warm-up /stretch program that's tailored to your specific activity and your body. In other words, there is no one size fits all guidance when it comes to stretching!!!