Internal storage is the storage you get by default with any device that you purchase. It is usually suffecient in most devices but some come with not enought space. However it is the first location users can turn to to store their data.
Every computer has a hard drive although most users do not see it. It is sold with the device and is where the operating system is installed. It is the users first location where they can store all of their data. They use a SATA interfacet to connect the computer.
Solid state drives are a spin of regular hard drives. But they have no moving parts, they take the flash storage method. Because they have no moving parts the they work a lot faster than regular hard drives. This allows the users to access their data on the fly. However because of their great performance they cost a bit more than regular hard drives.