Many of today’s computers from the big-name manufacturers come with multiple partitions on the computer’s hard drive. Your manual might refer to a "utility partition" or a "recovery partition" or even both, as well as talking about your C: drive. So, what are these things?
Terminology is part of the problem. Sometimes, "drive" means the physical piece of equipment. In Windows terminology, "drive" can also mean a partition that is formatted in a way that Windows can recognise it and read or write to it. For example, your C: drive is one such formatted partition. It could take up the whole physical hard drive, but it doesn’t have to.
Let’s take a simple example, where you have one physical hard drive in your computer. By "physical hard drive," I mean the piece of hardware that has some magnetic disks inside it and some magnetic reader/writer heads to actually read or make changes to the disk. Without partitions, this is a piece of useless junk — it is only valuable to someone who is going to write partitions on it and then "format" it for data.
The first step in getting a new (or replacement) hard drive ready to be used by your computer is to at least one partition on it. That is, to magnetize portions of the drive in certain ways that Windows (or Linux, or whatever Operating System you use) will know how to recognize.
At this point, all that we have is certain portions of the hard drive allocated into "primary partitions" and/or an extended partition. An extended partition can be divided into one or more logical partitions. Each of these, once it is formatted can hold programs and data and can be bootable, depending upon the operating system.
Let’s write one partition on it, taking up the whole physical drive. To be a bootable Windows partition, this partition has to be created as a "primary partition," so we’ll make it a primary partition as we create it.
At this point, we still have an unusable piece of hardware. A primary partition could be used for almost any type of operating system. Our next step is to "format" the partition — that’s where we write lots of little markers all over the partition.
These magnetic markers are designed so that the drive and the operating system can quickly figure out WHERE the read head is on the drive, synchronize with the formatting marks and the data, so it can actually read the magnetic patterns of data. Those are what we want — why we have the drives in the first place.
Why are partitions important? So you can segregate data and different OS installations from each other. Computer manufacturers use hidden partitions, that are not normally visible from Windows, to keep their recovery utilities and recovery data. If you use the recovery utility (or recovery CDs), you will usually replace the entire partition that has Windows on it with a factory-fresh installation.
Another reason for multiple partitions is so that you can install multiple operating systems. If you wanted to install Windows and Linux, for example ,you set them up in different partitions. Similarly, you could set up multiple versions of Windows or multiple versions of Linux and boot whichever one you wanted to boot.