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A partition table is like the table of contents in a book.
Partitioning a hard drive is analogous to being given an empty warehouse (your hard drive) for storing your stuff. You can either just drop all of your junk in the middle of it and have to root through it every time you want to find something, or you can divvy it up by building walls to segregate off areas that will hold a type of property ("Bikes go here, lawn equipment goes over there"). The partitions on a typical hard drive can provide a similar solution, usually a partition (or holding area) for the operating system, and another one for all of the programs. Some even take it a step further by creating another partition just for the data, so the operating system goes in one partition, the programs in another, and the actual data in yet another.