A referendum held in Springfield decrees that all illegal immigrants in the town will be deported. Homer Simpson discovers that Apu is in that category and decides to help him with his Citizenship Exam. The episode ( Season 7, episode23) also shows that Moe is completing the exam, for the same reason.

Moe Szylack is the proprietor of "Moe's Tavern" the bar that Homer Simpson and his cronies frequent. Moe's ethnic origins are ambiguous, other episodes have him as Italian, or Armenian.

Apu Nahasapeemapetilon is the owner of the Kwik-E-Mart. Despite having a PhD, he prefers to run this convenience store.

More Info: en.m.wikipedia.org