Jeltz is the captain of the Vogon fleet that destroys the Earth, and he is representative of the Vogons as a whole: he's cruel, bureaucratic, and he writes terrible poetry. Let's take those issues in reverse order (just to be as cruel as the Vogons).
(1) Vogon poetry gets a whole big scene to itself, so we can see how terrible and painful it truly is. (Where can we get some of those Poetry Appreciation Chairs?) The Vogons write terrible poetry not because they like poetry or even think it's good. They write it out of pure stubbornness: the "only thing that kept them going was sheer bloodymindedness" (7.4). It's pretty similar, really, to their reason for surviving without evolution: they have a "thick- willed slug-brained stubbornness" (5.3). They just keep on trucking.
The Vogons' terrible poetry helps show us one of their primary qualities: stubbornness. In that way, they contrast nicely with Ford Prefect, who is pretty flexible in his approach to life: when the planet he's on gets destroyed, he catches a ride; when he's about to be thrown out of an airlock, he tries to convince the Vogons about the wonders of culture, and so on. It's hard to imagine a Vogon doing anything but sitting down heavily on something in those situations.
(2) Similarly, both the narrator and the Hitchhiker's Guide note the bureaucratic aspect of the Vogons: not only do they fill the Galactic Civil Service, they also have a stubborn love of rule-following in all sorts of situations. "They wouldn't even lift a finger to save their own grandmothers from the Ravenous Bugblatter Beast of Traal without orders signed in triplicate, sent in, sent back, queried, lost, found, subjected to public inquiry, lost again, and finally buried in soft peat and recycled as firelighters" (5.63). As the list goes on, the requirements get stranger and stranger, but all of them point to the Vogons' love of doing things according to the rules. Remind anyone of Prosser? (For more on this, check our Politics section.)
(3) Our favorite example of Vogon cruelty is what they do to their own planet. On "Vogsphere," the ugly Vogons seem to take their frustrations out on their world and the plants and animals in it: the Vogons smash "scintillating jewelled scuttling crabs"; the Vogons cut down and burn "tall aspiring trees with breathtaking slenderness and color"; and the Vogons sit on and break the backs of "elegant gazelle-like creatures with silken coats and dewy eyes" (5.4). Too soon, Adams. Too soon.
Now, the first two actions might be reasonable: a Vogon has to eat, so crab meat cooked on a wood-fire might be nice. But the third is clearly just jerk-style behavior, since the Vogons don't get anything out of breaking the backs of these gazelles. It's just being jerks for the sake of being jerks. There's not even any bureaucratic reason to break the backs of the gazelles. It's not like we need broken backs to fill out bureaucratic forms on. They are just jerks.
One final note about the Vogons is that their name sounds like some generic alien species from any science fiction series. If we just listed alien species names—Daleks, Klingons, Vogons, Vratix, Zakdorn—would you be able to tell which alien was from the comedy series and which aliens were from more serious science fiction works?*
Is Adams making fun of science fiction conventions? Or is he saying that the Vogons are actually pretty scary, if you get right down to it, despite how funny or absurd they might seem? Stubborn, unimaginative, bureaucratic, senselessly cruel peopl—er, aliens—who wreck their own planet? Sounds pretty famili—er, scary to us.
*(Answers: Doctor Who, Star Trek, Hitchhiker's Guide, Star Wars, and Star Trek. Thank you, Internet, for all your nerd resources.)
The Dentrassi are a species of alien who cook for the Vogons. They are important in the book, since they pick up the hitchhikers Arthur and Ford, but we don't actually see a lot of them. (We could add, though, that they are motivated to work for the Vogons mostly by money, just like Benjy mouse and Frankie mouse and the philosophers (5.9).) It's not much different up in space from the way it is down here, huh.