When we talk about “drives” labeled C:, D:, and so forth, we’re actually talking about partitions, sections of the physical drive. Every hard drive in use has at least one partition. You can shrink that partition and create new ones out of the extra space. You’ll find this useful if you want to install more than one operating system, or if you want to truly separate programs and data.
But first, you need to take some precautions.