Do you realize your idea of building a system yourself might be a bit unrealistic in view of your obvious lack of knowledge? I mean, you did admit you are clueless. Do you really not know what RAM is? Tip: go to google.com and search on what is computer RAM?
And, yes, camerongray was probably implying more than one drive, a SSD (Solid State Drive) and a conventional hard drive. Since SSD are more expensive than conventional hard drives, some performance-oriented users like to use this dual drive configuration and put the OS and programs on the SSD for faster launch speeds, as camerongray stated, and use the conventional hard drive for storage of files (documents, photos, music, etc.).