Stan is the youthful conductor of the Knight Bus. Significantly, Stan is the first youth we've seen outside of a Hogwarts environment (well, besides Harry). If his speech is anything to go by (colored by slang and a thick, cockney accent), Stan is a lower-class wizard. But Stan is also a gossip, and gives us, and Harry, some vital intel on Sirius Black at the start of the novel. Based on his gossipy ways, Stan seems like the type who would spend his day reading Us Weekly and going to horror movies – he has a real relish for gossip and for dark stories that comes out in his Sirius Black spiel.
Overall, Stan is an information tool. But he also shows us a new side of the wizarding community by introducing elements of class to us. In addition, Stan ties into themes of community and memory; he speaks with authority about "common" knowledge events, even though he would have been a young kid when the Black incident happened. Finally, Stan provides us with comic relief after the tense scene in which Harry inflated his Aunt Marge and ran away from home (there's something you don't hear every day). Check out how he still calls Harry "Neville," the fake name that Harry gave, even well after he discovered that Harry is indeed Harry Potter (3.113).