HD stand's for high density. PE Stand's for PolyEthylene. HDPE slick's look & feel similar to Surlyn slick's.
The main difference being HD is not as soft or elastic as Surlyn. This makes the board slightly stiffer than Surlyn but mean's the board cannot bend as fast & thus does not have as much recoil when releasing.
HDPP can crease more easily & will crease deeper due to it no being as elastic. HDPP is usually used with PE Core's & low end PP boards as it is much cheaper & looks & feels very similar to Surlyn slick.