Kinda unspecified what info you want.
Going to assume you don't know what an SSD is.
Both SSDs and HDDs do the same job: They boot your system, and store your applications and personal files. But each type of storage has its own unique feature set.
If you wanna know more.
http://www.diffen.com/difference/HDD_vs_SSDYou can use both ssd and hdd at the same time. The ssd could be for Windows, programs and main games. While the hdd is for movies, documents and large games.