There are endless studies that say this, that, and the other about the optimal weight you should be lifting to reach your goals.
Here's the Mustard Fitness opinion:
If your primary goal is to increase your strength (your peak force), you should be going heavy.
That means working with a weight that only allows you to hit 1 - 5 reps per set. Take longer rest periods and aim to increase the weight used every week or two. Personally, when training for my deadlift strength, I go for 5 sets of 4 reps, with 3 minute breaks between sets. (the Mustard 5, 4, 3 System... not copyrighted)
If your primary goal is purely hypertrophy/fat loss (bodybuilding/looking incredible with all your clothes on the floor), you should also be going heavy, though not quite at a strength-training level.
Find your 'burn weight' with exercises. This means that you should aim to fail with a load somewhere between 6 and 12 repetitions. If you reach 12 reps and feel the ability to hit 1 or 2 more, the weight is too light and you should increase. Of course, if you can't hit 6 reps, the weight is too heavy and should be reduced a tad. Keep your rest periods between 60 and 90 seconds. Cue the red carpet.
If your primary goal is muscular endurance, then, you've guessed it, your reps need to be high and your weight low.
Aim for a weight that allows you to hit between 12 and 20 repetitions before failing. Reduce the rest periods here to 30 - 60 seconds and get all sweaty and intense!
I hope this helps!