The internal storage in desktop computers is a magnetic disk drive, and increasingly, a solid state drive (SSD).  Hard drives are mostly SATA drives.  See 
SATA, 
SSD, 
internal drive and 
magnetic disk.
Three-Trillion Bytes of Disk Storage
This 3TB internal hard drive was on sale at Micro Center in the summer of 2014.  (Image courtesy of Micro Center.)