Panarthropod is a term used to refer to the superphylum that includes the large phylum arthropoda as well as the slightly less prominent phylums onychophora and tardigrada. The use and purpose of a term such as "superphylum" is always a bit hazy, and in this case perhaps doubly so.
The arthropods, as a group, are extremely common, and economically important (both positively and negatively), and for that reason extremely well studied. The Onochophora, commonly known as the Velvet Worms, are mostly a curiosity, and the Tardigrada, or Water Bears are doubly a curiosity. There are many pressing concerns with the mainstream arthropods, such as colony collapse disorder amongst honeybees, that are probably keeping a slew of entomologists busy. In all of this, the possible ramifications of how the arthropods relate to their lesser known sister groups is probably not the most pressing concern. And yet it does provoke some questions.
Almost all true multicellular life sprang up fairly quickly (in geological time) during the Cambrian Explosion, or perhaps briefly before. Because of the quickness with which the groups developed, it is still not clear how the panarthropods are related to such groups as the annelids (true worms), mollusks, nematodes and a host of less-well known phylum. It was often supposed that the onochophora, which looks something like a caterpillar, was the link between the segmented annelids and the still segmented, but less obviously so, arthropods and tardigrades. It seemed like a likely story until someone went and invented DNA sequencing and other fancy stuff, and discovered that the surface anatomical similarities were only superficial. As far as I can tell, a lot of this DNA sequencing is fairly ambiguous and superficial, and there is no consensus answer on where and how the panarthropods split apart from the other phylum of the early Cambrian.
What does seem to be generally accepted is that the three groups are indeed a clade, all descending from a single ancestor, and that the anatomical similarities of the panarthropods are not merely the result of convergent evolution. However, there are two routes open to finding proof that they are a true clade, and also finding in which order the three groups developed, and from what: finding fossil evidence (which seems unlikely, since many of their ancestors were very small, soft-bodied creatures that died out close to 600 million years ago) and DNA sequencing (which seems to involve a number of technical issues that lead to unclear conclusions). Because of this, the complete story of panarthropoda may never be solved.