One of the main internal components of computers are hard drives. Your data and your operating system (Windows, Linux, etc) are stored on the hard drive. Anything you save, you save to your hard drive. It is the storage media for computers.
There are two main types of hard drive: Hard Disk Drive and Solid State Drive.
Hard disk drives (HDDs) are the most common at the moment. They consist of a stack of spinning platters and magnetic heads attached to a moving actuator arm. The arm moves the magnetic heads over the platters. Data is kept on the platters using magnetism. HDDs contain moving parts and can be damaged if severely bumped or dropped.
Solid state drives (SSDs) are becoming more common, and less expensive as technology advances. They use the similar technology as thumb/flash drives. Solid state drives contain no moving parts and aren’t easily damaged if bumped or dropped.
SSDs typically run faster than HDDs. This means your operating system and programs will load faster on an SSD than on an HDD.
Some people have multiple hard drives in their computers, using an SSD for the operating system and an HDD for data storage. Using the SSD for the operating system means the computer will boot in less than a minute (usually), which is quite a bit quicker than booting the operating system from an HDD. With this setup, programs can be installed on either drive, but if installed on the SSD, they will run faster.
Hard disk drives cost less than solid state drives but as technology progresses, the cost of SSDs is coming down.