From Homestar Runner Wiki
While most of the characters in the Homestar Runner body of work have physical characteristics unique to themselves, quite a few Poms have been depicted, suggesting that they are members of a well-defined species.
The overall body shape of Poms is quite consistent. Their bodies are large, spherical, and seem to be composed of a spongy matter capable of absorbing and containing objects (as seen in Where's The Cheat?, where Pom Pom keeps his cell phone inside his body, and again in hiding, where Strong Bad is seen hiding inside of Pom Pom). Poms' bodies are also very light, which causes Strong Bad to speculate in his email pom pom that Pom Pom may be filled with helium or hydrogen (though neither one of these explanations has any firm grounding in fact). Poms have short nubs for arms and legs, and their heads are ellipsoidal in shape with two small eyes but no other visible facial features. Pom Pom's girls are notable exceptions to the standard Pom anatomy, as they have typical Pom faces but human hair and bodies.
Poms are able to communicate, though with an utter lack of intelligible speech. Pom Pom himself speaks only through bubble noises, while his girls haven't been seen to communicate with anything more than giggles. Nevertheless, characters have no trouble understanding Pom Pom, so it is evident that Poms' language is discernible among themselves as well as by other species.
Poms come from the Isle of Pom (which looks like a medicine capsule), where they seem to organize themselves in traditional families and social structures (as seen in Pom Pom, Too, Pom Pom has both parents and neighbors). In Kick-A-Ball, it's revealed that Pom Pom has a cousin named Tim Tom, who has a very similar color scheme to the kickball used throughout the toon.