And HDD is a conventional hard drive that has spinning discs inside to hold all your data and are almost always slower then SSDs. And SSD is Solid State Disc. Which means no moving parts. Computers with SSDs benefit from having faster startup times, and programs load almost instantly. When copying a file to an SSD it's a lot faster then a HDD. HDDs are generally cheaper and SSDs while they have come down a lot in price over the years are still more expensive. Most laptops use a 5400rpm HDD where as desktop PCs usually use 7200rpm HDDs. Which means how fast the disc will spin when operating. Obviously a 5400rpm would be slower, but they claim to use the slower drives to save on battery. Hope that helps.