HDD stands for hard disk drive; it represents the conventional hard drive style in which an actual physical disk spins and stores data. This is less efficient because, in order to access data, there is always a slight delay while the disk spins. Compared to this, the SSD, which stands for solid state drive, is more efficient since there is no physical disk spinning inside. All the data is stored in microchips and since there are no moving parts, the SSD can load data faster, giving you maximum efficiency.