From Wikipedia, "A practical effect is a special effect produced physically, without computer-generated imagery or other post-production techniques."

Some of my favorite practical effect species are from TOS. There's the Horta from TOS "The Devil in the Dark" s1e25. I was about 9 years old when I first saw the Horta episode. Old enough to know it wasn't real, but young enough to be utterly fascinated by something that looked like an angry chunk of hot lava making its way around on the ground.

TOS's the Salt Vampire's look scared little kid me. Truth be told, the way it looks still gives me the creeps.

The Gorn (TOS "Arena" s1e18) didn't scare me, but I thought it was intimidating. Sure, it looks like somebody in a lizard man suit. But, that Gorn had a knife! And, was trying to kill Kirk! Bonus points for the Gorn's sparkly, stylish armor.

I think tribbles are just adorable.