HDD = Hard Disk Drive
This is the so-called hard disk. It consists of one to three disks (platters). This data is stored in the form of the magnetization of individual cells. For reading this information, the reading head senses the magnetization of the individual cells. In the notebook sector, there are two common speeds with which rotate the disks in the disk: 7200 and 5400 rev/min. This affects the access times and data transfer rates. The power consumption is in hard disks with 5400U/min is slightly lower than in those with higher speed.
SSD = Solid State Drive
An SSD consists of semiconductor memory devices. This requires no moving parts. Thus decreasing the power consumption and noise level. The data transfer rates much faster in contrast, while the access times (latencies) fall - which means that data is available virtually immediately, without having to read the header data first have to look on the magnetic disk. In addition, SSDs don't have mechanical parts taht can fail (see head crash ). The benefits are, however, currently still associated with a significantly higher price compared to normal hard drives.
SSHD = Solid State Hybrid Drive
Here a mixture of hard disk and SSD is used. The standard HDD is switched on a small SSD. These flash memory, the hard drive automatically loads in order to place frequently used programs and files for a faster availability after a certain "learning time". Hybrid hard drives are seen priced to find something above a normal HDD, but far less than SSD's. These hybrid hard drives were previously sold by Seagate under the name "Momentus XT". Now Hybrid Solid State Disk are widely available for both desktops and notebooks. Despite the name change, these hybrid hard drives still offer the same features and the same high performance. Under the name "Thin Laptop SSHD" there is a 7mm height variation, which thus fits in Ultrabooks.