What is the Difference???

Aside from the obvious price difference, the short answer is HDD has many moving parts and generates heat, SSD has no moving parts therefore no heat is produced and much faster access times.

Normal hard drives use a rotating metallic platter to store information. When information from a specific location is needed, the drive must rotate to that location before it can be read or written. The drives rotate very quickly but computers operate much much faster, thus the use of mechanically moving parts adds a significant delay between when an operation is requested and when that operation is completed.

Solid State Drives or SSDs for short store their information electrically through the use of transistors, thus removing the movement delay. This greatly reduces the time delay for read and write operations which can make certain applications more responsive.