The Asus UX51VZ has an SSD storage medium which is better than most laptops that have been around for more than a year. Most similar spec laptops that have been available for around the same time as it have an HDD storage medium. Solid State Drives (SSD) store data in semiconductors on a circuit board rather than using magnetic or optical media. Solid State Drives have no moving parts which makes them very fast and reliable.
An SSD storage medium is in fact the best storage medium amongst all notebook PCs of a comparable age and spec to the UX51VZ.
The worst storage medium amongst similar types of laptop is HDD.