Using tú and usted:
familiarity isn't always welcome

Getting too familiar?

Spanish, like most other European languages except English, has two forms of "you", the formal and the familiar.  In Spanish, tú is the familiar form; usted (abbreviated Ud. in writing or sometimes Vd.) is the formal one.  Tú is often taught as the primary form in school Spanish classes in the U.S., since children and teenagers do use tú with each other all over the Spanish-speaking world.   Some "heritage speakers" of Spanish are limited to the use of tú since their experience with the language has been almost entirely within the circle of family and close friends.  On the opposite extreme, in some traditional families, children and parents use usted with each other.

The use of tú is governed by a complex set of "rules" and conventions that vary from region to region, and even within regions.  As a general rule, tú is used with small children, and among close friends of equal socio-economic status.  Also as a general rule, the use of tú, except in situations involving students or the like, is subject to mutual expressed consent; with the person who is older or in a "higher" position having the prerogative to initiate the use of tú.  While traditional norms are changing throughout the world, there are still plenty of Spanish-speakers who place great emphasis on the traditional forms of politeness.

A person who uses tú with another person who might be perceived to be in an inferior position, such as a taxi driver, runs the risk that the person so addressed will feel "put down".  While nothing will probably ever be said, the atmosphere and perception of the person who used tú won't be very good.  Conversely, initiating the use of tú with a person who by virtue of age or socio-economic position believes they are the ones who have that right can also sour the atmosphere.  Inappropriately using tú with somebody like a Customs agent or Police officer can have downright unpleasant consequences.

The best general rule for non-native speakers to follow is, with the exception of small children or somebody of the same age and condition you might meet at school or in a social environment, to always use usted on first meeting a person.  Nobody will ever feel insulted or slighted by being addressed as usted; and if they want to use tú, it is easy and painless to say so.   It is easy for Americans particularly to overlook what we tend to see as silly, old-fashioned customs; but the potential harm is great enough that it's worth while to respect and observe them.

Like the Latin custom of shaking hands on every meeting, even multiple times in one day, this is different and strange enough for Americans that it requires constant practice.  The better your Spanish is, the more important such "niceties" are; if you speak fluent Spanish and commit these errors it is likely to be perceived as deliberate rather than accidental.