what is the difference between the two and which is better ?
The technology. SSD's use NAND Flash to store data while hard drives use magnetic platters.
The end results, SSD's are about 5 times faster overall (instant access and great random I/O), more durable, consume less power, and less prone to failure (lack of mechanical parts), and silent. The downside: Price and capacity. SSD's don't store a lot, the cost quite a bit, especially for the larger capacities. While prices of SSD's have plummited, they are still much more expensive than hard drives on a price per GB scale.
Personally, SSD's are better, no comparison. The only time an SSD wouldn't be better is if you are archiving media where the extra performance isn't neccessary and greater capacity is important. Most enthusiasts and gamers go with a 2-drive configuration: SSD as boot drive (operating system and essential apps) and a mechanical hard drive as storage for non-essential applications/games, and other media. A good affordable size for SSD's are 120-128GB.