Bubotubers are very ugly; they look more like giant squirming black slugs than an actual plant. They protrude vertically out of soil and have a large number of large, shiny swellings on them, which are filled with pus.
From the Story
Discovered in Book 4, Chapter 13, Mad-Eye Moody
"The fourth-year students are instructed to collect the pus from the Bubotubers during their Herbology lesson. By squeezing the swellings on the Bubotuber plant, the students are able to bottle the pus for Professor Sprout."