Oh, bloody. I just lost the whole thing that I had typed.
I'll explain partitioning in exchange for a picture from Disk Management. Go to Control Panel - Administrative Tools - Computer Management, and select Disk Management on the left.
A brand-new hard drive is a big bucket of unusable storage. To make the storage show up as one or more disk letters, you have to create one or more "partitions." A partition is a chunk of that bucket of storage that is now available for use. Well, not quite.
After you create a partition, you have to format it. This lays down the outline of the "file system," which contains the folders and file names, and keeps track of where the file data is stored.
Most drives that you have seen will have one partition with all the space on the drive in it, so the drive letter shows the same size as the drive. The previous poster suggests that this drive may have more than one partition, or have one small partition and leftover unpartitioned space.