All the powerful video cards and processors in the world won't make a difference for a computer if it lacks a hard drive. As the primary storage device, these drives record the data of a computer in multiple ways ranging from traditional hard drives (HDD) with internal components to solid state drives (SSD) with no moving components at all.
Traditional internal hard drives have the potential for mechanical failure, but are far more tolerant of being rewritten than solid state drives are. For solid state drives, the reverse is true, and they often have a smaller storage capacity than their siblings. Each kind of hard drive comes in mountable internal forms suitable for specific computer types, or mostly universal external drive form.