Informal expressions are casual phrases or words used in everyday conversation. They often reflect a relaxed tone and can include slang, idioms, or colloquial language. These expressions help convey emotions and create a friendly atmosphere, making communication feel more personal and relatable. Common examples of informal expressions include phrases like "What's up?" or "I'm feeling blue." While they are widely understood in casual settings, they may not be appropriate in formal situations, such as business meetings or academic writing. Understanding when to use informal expressions is key to effective communication in different contexts.