That´s a very common method to intensify your workout. Unfortunately it´s used way too often.
A forced rep is a a rep at the end of hard set, you wouldn´t be able to complete, where your training partner supplies you with a slight ext´ra lift. The impact on your muscle is very intense, because you´re training at an unusual high level. The problem with this method is, that doing frequently can cause a permanent overtraining. Also it´s difficult for your training partner to recognize how much of the weight you are capable of doing and how much support you need.
Another concept of doing forced reps is to do a heavy set to failure, take just a very short break and then do another rep. Then again rest a few second and do another.

Of course, usually you shouldn´t cheat and instead take care of a good technique. But as the working muscles fails you can sometimes give them a little support from the stabilizing muscles. Imagine to do heavy barbell curls and at the end of the set your upper arms are not able to do one more correct curl. At this point you begin to use your shoulders and back to support the biceps and so squeeze out another 3-5 reps. But be carefully, cheating doesn´t mean to have an uncontrolled weight swing or do half of the reps this way. Use it only casually to get a little more intensity in the working muscle.

That´s a very smart way to intensify your workout. Supersets are two exercises performed without resting time, in some cases even three exercises. Besides a good musclular workout you´ll have a challenging condition training. This extra intensity could be a limitating factor in the beginning.
There are two ways to do supersets. You can do two exercises for the same muscle in a row, such as bench press and dumbbell flys. Or you train two different muscles, preferably agonist and antagonist, wich creates a tremendous pump in that area. But it works with two completetely different muscles, too.

The stripping method is about reducing the weight, as you fail at the end of a set. If your muscles are too tires to do one more rep, quickly reduce the weight by picking another one or taking off a few plates. Depending on your training schedule you can do this several times until weight is very low. Have a training partner aside to keep the resting time at an absolute minimum.
Like supersets, this method is also very exhausting and can be limited by your condition.

Extended sets relate to the stripping method, but with constant weight. All you need is to perform a heavy exercise, let´s say bench press. After failing have a short rest of a few seconds and then bench press again, same weight, less reps. This can be done several times and therefore is a very intense method. Beware of overtraining.

With this method you can stimulate a maximum ammount of fibers in the target muscle. Think of the following situatuion: You´re doing a compound exercise like bench presses, training chest, triceps and shoulders. The chest is by far the biggest and strongest muscle among these three. That´s why sometimes you have to end the set , because triceps and shoulders fail before the chest muscles. To compensate for this, do an isolation exercise in the beginning, like butterfly or dumbbell flys, wich pe-exhausts the pectorals.
Let´s summarize: simply have an isolation exercise for a large muscle group before performing a heavy compound exercise. That works for every major muscle.

At the end of an exercise, when you are too tired to do do one more rep, continue to do a few partial reps. Here´s an example: You´re doing lateral pulldowns and at the end of the set you´re too tired to perform another full-range rep. Of course you´re still able to do a few reps at the top of motion, so continue until complete failure.

Isolation training is a good way to train weak areas or to develop specific muscles. Simply do a few exercises that isolate the target muscle in a row. To have your chest trained really hard without hitting triceps and deltoids, do a few fly-exercises like dumbbell flys, pec deck butterfly, negative or positive dumbbell flys. Keep in mind that you might create imbalances over the time.

Another possibility to overcome weak points or develope a specific muscle. As you´re doing your exercises for, let´s say, back, but you want to give your calves an extra workout, just do it in between the back exercises. The other day continue with your normal calf training. That method works best with smaller muscles. Training back and mixing it with thighs wouldn´t be that smart.

Whenever you´re extending the working musles you´re doing negative movement. While doing that you put more strength on the supportive structures like tendons and joints and therefore increase their resistance.
Negative reps shouldn´t make up your complete exercise. Have a few at the end of a set. You can try to cheat the weight up and than lower it slowly and controlled or let a training partner support you. Never let the weight drop.

The names comes from 3x7. It means that you do seven half reps in a row at the elower part of motion, another seven at the upper part and conclude the set with seven full reps. Of course, it also works with 8,8,8 or slightly more. The training effect here comes from the surprising movement that´s very unusual for your muscles.

The purpose of this method is to stimulate a certain musle in all different variations and angles. Don´t confuse it with supersets. Multi-exercise sets are done with a normal resting break, but just one set of each exercise. For example do one set of triceps pushdowns, then triceps extension, close grip bench press followed by kick-backs and so on until you have absolutely exhausted your triceps.