Energy balls: The ability to throw spheres of energy that resemble electrical discharges.
Super Strength: The ability to possess strength far beyond humanly possible.
Shimmering: The ability to teleport through a "shimmer", a demonic form of teleportation.
Telekinesis: The ability to move objects and beings with the mind.
Incineration: The ability to incinerate a being with a gesture. Belthazor used this power once to set a demon on fire and vanquish him.
Apportation: The ability to teleport objects. Belthazor used this power to swap his and Phoebe's books in a book store.
Shapeshifting: (limited) The ability to alter one's physical shape. Belthazor could shapeshift between his demonic and human form at will.
Sensing: The ability to sense the location of other beings.
Adjusting: The ability to fight through inhibitory molecular powers.
High Resistance: The ability to be highly resistant to physical and magical harm and survive otherwise lethal attacks. He was one of the few demons who couldn't be vanquished by his own energy balls and was able to survive Piper's power of Molecular Combustion even in his human form.
Immortality: The ability to possess an infinite lifespan and an arrested aging process.
Through spell, potion, power stealing, etc.
Illusion Casting: The ability to cast illusions. Cole mentioned having learned a trick from the Demon of Illusion and appeared in Phoebe's favorite movie to let her know he was back.
The Book of Shadows contains a potion specifically to vanquish Belthazor. However, this contradicts his entry, which states there is no known way to vanquish him. However, since the book is passed down through generations, the potion could have been added later.
Belthazor has the most pages in the Book of Shadows dedicated to him, having a total of four. The Cole Turner page was written by Phoebe, his main page was written by an unknown ancestor (certainly born before Penny, who created the vanquishing potion).
Belthazor has appeared in a total of 10 episodes and 2 comic issues throughout the course of the series.