Capacity: The amount of information that can be stored on a hard disk. For example, a 1TB hard disk can hold approximately 250,000 photos taken with a 12MP camera or 250 movies.
Rotational Speed (rpm): The most important feature, along with capacity, is the faster the rotation speed of a hard disk, the faster the access and storage of data it can provide.
Cache Memory: Intermediate temporary memory between the disk and the computer, allowing the hard disk to provide more immediate and faster access to data.