The difference between 'tú' and 'usted' is usually thought of as a difference in formality. One uses 'usted' in formal-level conversations and 'tú' in informal. North Americans generally prefer informality (use of first names, non-use of titles, etc.). Consequently, North Americans sometimes want to use 'tú' thinking that it is a sign of informality when in actuality 'usted' would be more appropriate. The problem is that in addition to formality and informality, the use of 'tú' and 'usted' also signify respect (similar to 'sir' and 'ma'am' in the southern portion of the United States.). As such, Mexicans, for example, use 'usted' to show respect more than simply to display formality or informality. Note, however, Roberto Salmón's observes that 'tú' is used much more today among young executives than what it was 30 years ago. He goes so far as to say that in this respect they are becoming Americanized. A good rule of thumb in professional situations is to begin with 'usted' and wait for your Latin American counterpart to invite you to use 'tú'. It does vary from one country to another, not to mention the use of 'vos' that is prevalent as well.