A hard disk (also called a "hard drive") is much like a filing cabinet. The programs and data are stored on the hard disk and the computer accesses them as needed. When the computer accesses the hard drive, it is reading the stored information into memory. That memory is the temporary workspace. The original file is still on the hard disk and is left undisturbed. When the computer stores information, it writes the data to the hard disk. That process results in the old file being replaced or modified with the new information. If you save data to a new file, or install new software, the information is written to the disk in an available, unused portion of the disk.
Most hard disks are installed internally to the PC, although external models are available. Also, a hard drive can be installed using a modular bay to make the hard disk portable.
The hard disk contains a recording media onto which data is written magnetically. That media is the platter and it spins at high speed. Hovering just over the surface of the platter are the "heads". Those heads read and write data to the platter. When you hear a whining sound, that is the drive spinning up to full speed. When you here a clicking sound those are the heads moving to read or write data.
Older versions of Windows 95 cannot address more than 2GB on a drive. Therefore, it is necessary to partition larger drives into multiple smaller drives so that Windows can use the space. In later versions of Windows 95 and all subsequent upgrades (Win98, Me, 2000, XP), larger drives are not a problem.