"Aller" is the infinitive form of the verb which means "to go". You use it when you need to use the infinitive. Example: Je veux y aller. I want to go there. "Allez" is the the second person conjugation of "aller", used with the subject pronoun "vous" ("you", when speaking to more than one person, or to one person to whom you should speak with noticeable respect). Example: Vous allez trop vite. You're going too fast.