Straight dielectric grease is a silicone based product. Thermal heat sink compound or thermal paste has a similar base but have fillers(often zinc oxide) to give it superior heat transfer properties.

Dielectric grease is clear, while heat transfer compound is usually white or silver due to the fillers.

I use dielectric grease to protect wiring connections from oxidation, such as battery terminals - as a topical treatment after I tighten the connection. I also use it to protect and rejuvenate rubber products such as carburetor and spark plug boots.

I use the thermal compound between my voltage regulator and it's heat sink as well as on spark plug threads. The trick with this stuff is to use it sparingly. More is not better. It also makes a big mess if you get it on other stuff.