Structurally, a hard disk drive (HDD) mainly consists of a set of spinning platters and a need-like "head". HDD stores all its data on spinning platters. It reads and writes data with the head. During read-write operation, the platters have to spin to bring the reference location, where data is stored (read) or is going to be stored (write) under head. The spinning processing and task of write/read operation decides speed of a HDD. IBM introduced first commercial HDD in 1956.
On the other hand, solid state drive (SSD) do not have any moving parts. It rather uses microchips for data storage. Absence of mechnical parts (i.e. spining platters and needle-like head) makes SSD much faster than HDD.
According to lastest report by TrendForce, in year 2016 one third of sold personal computers (PC) will come with SSDs. In the same report, SDD growth has been predicted to reach at 55 percent in year 2018.
PCs with SDD are much faster and more responsive. However, SSD is almost eight times expensive than HDD and has relatively shorter life.
In future HDD will be more effective for long term storage of large data files (e.g. photos) and backup storage.