SSDs are very expensive on a cost per GB basis, so you really want to use them only for files that are performance-critical. For most users this means putting the OS and your applications on them. For those files, the vastly shorter access times of an SSD (50 to 100X faster than a typical hard drive) mean that reading the many relatively small files needed to boot the OS or start an application program is much faster.
So if I play games, save the program on the ssd and the saves on the hd?
Games take up so much space, will it be worth it buying a ssd?