Teaching styles and classroom attitudes vary widely and are influenced by many different factors. Even where tradition does dictate professorial or student behavior, the patterns may not be evident to someone coming from a different tradition. Professors are the core of the teaching staff at most institutions in the United States. Full professors generally teach lecture courses, seminars, and courses for graduate students and upperclass undergraduates. A professor's informal style of dress or speech must not be taken to mean that he or she has a relaxed attitude toward assignments, class attendance, or the quality of your work.
Teaching assistants are most often graduate students pursuing a masters or doctoral degree.
You should feel free to approach them with your questions indeed, you will probably be on a first-name basis from the beginning of the course and you should not hesitate to ask their advice about course work, grades, or other matters. Since research universities began operating in the United States, graduate students have assisted professors by teaching undergraduate students. In many fields, the experience and training gained as a TA are viewed as necessary preparation for becoming a faculty member. TAs may be in charge of lab and discussion sections, or they may be required to grade the homework and exams given by the professor. In some cases, TAs may give lectures.