HDD and SSD
HDD stands for Hard Disk Drives, a hard disk drive is what stores all your data. It houses the hard disk, where all your files and folders are physically located. A typical hard disk drive is slightly larger than your hand, yet can hold over 100 GB (Gigabytes) of data. The data is stored on a stack of discs that are mounted inside a solid encasement. These discs spin extremely fast (either 5400 or 7200 RPM (Revolutions Per Minute) ) so that data can be accessed immediately from anywhere on the drive. The data is stored on the hard disk drive magnetically, so it stays on the drive even after the power supply is turned off. SSD stands for Solid State Drive, which can also be called a Flash Drive, is a type of hard drive. Though the architecture of a Solid State Drive is quite different from traditional Hard Drives, the name is carried over. An SSD uses a special kind of memory chip with erasable, writeable cells which can hold data even when powered off. It might help to think of an SSD as the larger cousin of the Memory Stick.
SSD vs HDD 7200rpm real comparison -Samsung 830 - WD 10EALX-