A block (pulley) is a wheel on a shaft mounted in a housing used for lifting objects or changing the direction of effort. Block and tackle refers to using multiple pulleys, one fixed and the other moving to create a mechanical advantage. Count the number of times the line passes not including the line that is pulled multiplies the advantage, but, it requires the same multiple of the amount of line that must be pulled through the system. For instance, a tackle with four passes of the line can lift 200 pounds of load with only 50 pounds of effort, but, it will require 20 feet of line to pass through the tackle to lift the load 5 feet.
Image by Welkinridge - Own work by uploader based on a diagram in "Notes on cargo work" by Kemp and Young. 1977, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4227455