HDD, which stands for Hard Disk Drive is a storage drive that uses a spinning disc to store data. It can be both externally and internally. SSD, which stands for Solid State Drive uses circuits to store data. An SSD doesn't contain any spinning discs. A close match to an SSD would be a flash drive. Most modern computers contain two separate slots for both SSD and HDD. You're probably already acquainted with an HDD slot. And SSD's is a thinner slot with a lengthy free space to place the SSD. As for their speeds, and SSD is a very fast storage drive. This is because of the lack of a disc. On an HDD, data is fetched by rotating the disc and finding the sector to fetch the data from, which doesn't happen on the case of an SSD. Hence the speed.