There is also something else.

There is fixed or variable variations on each schedule. Fix schedule keeps the same interval or ratio, 5minutes or 10 pushes. Variable is a vary of time or ratio, yet it averages. Lets say sometimes you change it to every 2 minutes instead 5 or 8 minutes, but it averages out to 5minutes. Here are 4 possible schedules:

Fixed Interval- The reward is presented based on time and the time interval is the same, always.
Variable Interval- The reward is based on time and the amount of time varies, but averages to a specific time.
Fixed Ratio- The reward is based on the amount of times the bar is pushed and the number is the same, always.
Variable Ratio- The reward is presented based on the number the bar is pushed and the number varies but averages to a specific ratio.