A partition can be thought of as a division or "part" of a real hard disk drive.
A partition is really only a logical separation from the whole drive, but it appears as though the division creates multiple physical drives.
Some terms you'll see associated with a partition include primary, active, extended, and logical partitions. More on this below.
Partitions are also sometimes called disk partitions and when someone uses the word drive, they usually mean a partition with a drive letter assigned.
In Windows, basic hard drive partitioning is done via the Disk Management tool.
See How To Partition a Hard Drive in Windows for detailed steps on creating a partition in each version of Windows.
Advanced partition management, like expanding and shrinking partitions, joining partitions, etc., can't be done in Windows but can be done with special partition management software. I keep updated reviews of these tools in my Free Disk Partition Software list.
Keep reading to learn more about why you might build partitions and to understand the different types of partitions that can be created.