Most training programs available online will give some simple progression from week to week such as 3 x 12 in one week and 3 x 10 the following week.
Progressions like these do not give you any real idea of what weight you should be using. Basing the weights you use off a percentage of your one-rep max (1RM) or Rate of Perceived Exertion (RPE) are two of the most common ways for ensuring you accumulate the correct amount of stress over a training cycle.
RPE involves basing the weight you use or number of reps you complete based on how difficult. There is nothing wrong with RPE in theory, the main problem with basing your weights of RPE is that often people will either push themselves too hard or not hard enough.
Percentages are much better for keeping people accountable. Using a percentage range such as 70-75% allows people to still choose weight based on how they are feeling while making sure they use an appropriate amount of weight.
There are many forms of periodization such as Linear, Conjugate and Daily Undulating periodization. We generally use 8-12 week training cycles using a linear-styled progression with a deload in the middle of the cycle.
You can get an idea of how we generally program our training using percentages in our Intermediate Powerlifting Program post