What is the difference between SAS and SSD hard disk? How will you use it in application level?

SAS is good performance drive where as SSD is extreme performance drive used for very high I/O intensive applications. 

Generally we use SATA/NLSAS for low I/O applications like file server.  SAS for databases that need better I/O. Finally SSD for exchange and db logs etc which require extreme performance (only Iin large organizations)