"Du" is the informal second-person singular pronoun in the German language, used to address friends, family, and people of the same age or younger. It conveys familiarity and closeness, making it a common choice in casual conversations. In contrast, the formal pronoun "Sie" is used in professional or respectful contexts. The choice between "du" and "Sie" can reflect social dynamics and relationships, and switching from "Sie" to "du" often signifies a closer bond, sometimes referred to as "per Du." This practice is common in German culture and language.