Hard drives are memory devices used for storing and retrieving information. Hard drives typically store the operating system and user files. This information is then loaded into other memory devices, such as RAM, or flash memory for use by the user.
Hard drives are typically internal components to a computer, connecting directly to the motherboard through a SATA or IDE interface. But it's not at all uncommon to have an external hard drive which connects through USB, Firewire or eSATA. These types of drives are commonly used for backup.
Hard drives may be partitioned so that an operating system sees the disk as more than one device. This is a commonly used technique for keeping system files separate from user files, or for installing multiple operating systems.
Conversely, hard drives may also be put in a RAID, which will cause an operating system to see multiple drives as one device. This is commonly used to prevent data loss in the event of a drive failure.
Most hard drives today are mechanical drives that read from a spinning disk. But increasingly, more drives are solid state drives (SSD), which use flash memory. These are much faster than mechanical drives, but typically they're more expensive and have a smaller capacity.