SSD : We’ll make no assumptions here and keep this article on a level that anyone can understand.  You might be shopping for a computer and simply wondering what the heck SSD actually means?  To begin, SSD stands for Solid State Drive.  You’re probably familiar with USB memory sticks, SSD can be thought of as an oversized and more sophisticated version of the humble USB memory stick.  Like a memory stick, there are no moving parts to an SSD, information is stored in microchips.  Meanwhile, a hard drive uses a mechanical arm with a read/write head to move around and read information from the right location on a storage platter.  This difference is what makes SSD so much faster.  As an analogy, what’s quicker, having to walk across the room to retrieve a book to get information or simply magically having that book open in front of you when you need it?  That’s how an HDD compares to an SSD, it simply requires more physical labor (mechanical movement) to get information.

HDD : Hard Disk Drives, or HDD in techno-parlance, have been around for donkeys years relative to the technology world.  HDDs were first introduced by IBM in 1956, yes folks this is nearly 60-year old technology, thank goodness vacuum tubes for TVs didn’t last so long!  An HDD uses magnetism to store data on a rotating platter.  A read/write head floats above the spinning platter reading and writing data.  The faster the platter spins, the faster an HDD can perform, typical laptop drives today spin at either 5400 RPM (Revolutions per Minute) or 7200RPM, some server based platters can spin at up to 15,000 RPM.

SOURCE :storagereview.com