Implication of using an SSD hard drive compared to a conventional one

Hi, being a hardcore game, i have recently started hearing about the use of SSD hard drives to speed of the processing speed in laptops and desktops. What is the meaning of SSD hard drives. Can anyone explain to me the mechanics behind this drive as compared to a conventional hard disk. Also i would like to know how is the significant increase in processing speed attained. I have been keen to buy an Alienware laptop and most of them are equipped with hybrids of both normal and SSDs. Can i know why is the SSD components so expensive? Is it significantly better than normal hard drives?
