That's because Spanish, like many other languages, has several different ways that you can address other people, all of which can be translated by the English word "you."
First and most obvious, there are singular and plural forms, which aren't distinguished in the English word, although we can usually tell which is meant. But more important to learning how the Spanish language is used, Spanish also has formal and informal (also called "familiar") ways of saying "you," the usage depending on the person you're talking to and/or the circumstances. The difference doesn't come across in translating to English, but if you use the informal "you" where the formal is required, you run the risk of sounding presumptuous or even arrogant.
First, here is a chart showing the various ways of saying "you" as the subject of a sentence:
- Singular informal: tú
- Singular formal: usted
- Plural informal: vosotros
- Plural formal: ustedes
- Katrina, ¿quieres tú comer? (Katrina, do you want to eat?)
- Señora Miller, ¿quiere usted comer? (Mrs. Miller, do you want to eat?)
- Katrina y Pablo, ¿queréis vosotros comer? (Katrina and Pablo, do you want to eat?)
- Señora Miller y Señor Delgado, ¿quieren ustedes comer? (Mrs. Miller and Mr. Delgado, do you want to eat?)
One reason for knowing which form of "you" to use is that there are corresponding words for "your" and "yours" as well as for "you" when it isn't the subject of a sentence. Also, Spanish uses different verb forms for the informal and formal usages, with the informal forms taking a second-person conjugation while the formal forms take a third-person conjugation (the same used for the equivalents of "he," "she," and "they").
The basic rule: As a general rule although keep in mind that there are exceptions when speaking with one person you can use the informal forms under roughly the same circumstances where you can use a person's first name in English. Of course, when that is can vary with age, social status and the specific culture you are in.
More specifically, the singular informal tú is used when speaking with family members, children, pets, friends or close acquaintances, while usted is used when speaking with others. In most types of Christianity, at least, tú also is used when addressing God in prayer. Tú can also be used contemptuously when speaking to a stranger.
Obviously, the use of tú suggests a certain amount of intimacy. But the degree of intimacy varies with region. In some places, people of similar social status will start using tú upon meeting, while in other areas doing so might seem presumptuous. If you're uncertain which to use, it is usually better to use usted unless or until the person starts speaking to you using tú, in which case it is usually OK to reciprocate (unless you're a child or the person speaking to you is a police officer or other authority figure). Spanish even has a verb, tutear, meaning to address someone using tú. In a few areas, as in much of Colombia, usted is used even with close friends and family members.
Plurals: In Latin America, the informal plural, vosotros, is seldom used, even when talking with family members, so ustedes is used in plural cases. In Spain, vosotros is generally used as the plural of tú.
The pronoun vos: In some parts of Latin America, the pronoun vos replaces or partly replaces tú, especially in spoken Spanish. In some areas, vos implies even greater intimacy than tú does, and in some other areas it has its own verb forms. As a foreigner, however, you're perfectly safe using tú even where vos is common.