Friends gather just to be together. Someone with a problem goes to a friend or family member for help at any time. One tolerates a lot from someone who is a friend. Friendships are only with the same gender. Friendships are with people of the same age.
Friends are available at any time.
People who don't know each other maintain a formal relationship. In general, people in the United States are very informal about titles and status. Often instructors will tell you on the first day of class what they would like to be called. You may also ask them how they would like to be addressed. Graduate students, especially graduate assistants and TAs, are more likely than undergraduates to be on a first-name basis with their professors.