Reps are something people usually neglect in terms of putting thought into how they are going to do them. Most people usually think: ok bench time, 3 sets of 10 i wanna lift this much weight, then start lifting.
If you want to achieve a certain goal, doing your reps in a certain way with a certain tempo will get you there quicker.
Form? Form? If you don't know what form is it is a way to describe how well you train. It includes rep tempos and co-ordination. Range of motion, positive, isometric and negative reps are all important.Another aspect i don't cover below is momentum but when doing exercises (unless they are full body explosive movements) you want to minimize momentum as much as possible. You are workout your muscles, not your ego. Weight is secondary to how well you can do the exercise.

RANGE OF MOTION
Range of motion is from the point of an exercises maximum stretch, to it's maximum contraction. Example: for bench, everyone's favorite lift, the maximum stretch is the point where the bar TOUCHES your chest, the maximum contraction is point of lockout of the minimum distance before. Depending on your goals you wither complete a lock out with you joints of not. If your goal is power or strength then lockouts are great, especially on the stretch because your muscle has to work MUCH harder to complete a rep if it's been stretched while in hypertorphy, it's all about time under tension so JUST BEFORE lockout is a good way to go.
 

TIME UNDER TENSION
This is as it sounds, it's putting your muscle under as much tension or strain as you can for the longest amount of time. Drop sets, super sets, giant sets, burn sets are all amazing bodybuilding tools. This is mostly for hyperrophy and doesn't serve much purpose in powerlifting outside accessory work. When you lock your joints out say doing a pullup, when you let your shoulders get pulled way up to your ears and straighten your arms when you come all the way down on the rep, all the tension is taken of your back. This doesn't mean stop half way down because that's cheating and not full reps. but stretch the muscle being worked as much as you can to the point JUST BEFORE lockout. Another example would be while curing. Instead of locking your arms out every rep (making your arms straight, get to the point just before then spring back up for another rep. Straighten your arm right now then relax it. Where your arm naturally goes to just before lockout is the difference between time under tension and breaking that time under tension.
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POSITIVE PORTION
This is the point of the rep everyone focuses on, the push of the bench press the pull of the pullup the standing up of the squat. It is an important potion of the rep but it is actually the WEAKEST portion of the rep. That;s right, you are weakest when you complete a rep, who'd have thought? My recommendation is regardless of your goals, a nice powerful positive rep is always a good idea. If you are doing an isolation exercise and you purely want size then a slower 2 second positive rep might be your choice but personally, I always do as powerful and fast positive reps as I can, UNLESS I'm trying to focus on a muscle and consciously squeeze it.

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STATIC/ISOMETRIC PORTION

This is the portion of the rep barely ANYONE trains. Isometric contractions are where your muscles are under tension but not moving. The static part of the rep is the peak of the contraction.  If you are doing chest flies on a twin cable machine, when you bring the handles together, if you hold them together and squeeze the chest every rep for 1 full second, you'll feel it. Believe it or not, the static is STRONGER than your positive. Incorporating some isometrics into your reps is a fantastic way to raise the intensity, it's amazing how a 1 second old every rep can make things harder several times over. Do some pullups and hold it at the top for a second every rep, do it with bicep curls, rows, especially cable crossovers, reverse flys and anything which will be hard to hold the peak contraction. This is great I highly recommend doing these in some if no ALL your sets.​
Isometrics can be used as a training tool during any portion of a rep but i prefer peak contraction simply out of personal preference. Half way up the rep is my second favorite but I find i have to purposely plan for that while at the peak contraction is a no brainer, you can do it any time.

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NEGATIVE/ESENTRIC

This is the final part of the rep. This is the lowering down of the bar on the bench, the lowering down of yourself during pullups, the squatting of the squat or the putting the bar down of the deadlift. The negative is (obviously) the opposite of the positive, it is the portion of the rep where your muscles being worked go from peak contraction to stretch. People drop weight all the time and focus purely on the positive, WRONG! The negative is your STRONGEST portion of the rep. In my opinion, a medium to slow negative is always a must unless you are doing something explosive like cleans or jumps or fitness like burpees and mountain climbers. Lower the wight down in a controlled manner for a count of 2-4 seconds and I can promise you, you'll have worked harder for longer and you'll get stronger quicker!

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FORCED NEGATIVES

This is AMAZING! the pump and the pain is off the scale! When you finish a set and your muscle is completely fatigued and you can't do another rep, instead of dropping the weight, you keep the weight the same and either get someone to help you complete the positive portion of the rep for about 3-5 reps or you jerk it up and lower the weight down EXTRA SLOWLY. A count of about 4-6 seconds is a good window to shoot for. Example: you just strained and grunted out a set barbell curls, you got 12 reps, you're red in the face, you're sweating and you can't even move the bar anymore. At that point jerk the weight up by using momentum, your legs and your back as help to complete the rep, then lower it down as slow as you possibly can, fight it with everything you have 3-5 times. This kills, it's hard, it never gets easy, its a golden training tool. This will help with building size because it extends the time under tension and it will get you stronger too! Double win!​