Spanish has an undeserved reputation for being easy. It seems to confuse even native speakers. People in Spain, for instance, can be very unclear about the distinction between tú and Usted, and people in Latin American are quite fuzzy about vosotros. For foreigners learning the language, it can seem very confusing since teachers and or natives will contradict each other, depending on what part of the Spanish-speaking world they come from.
This need not be the case. The distinctions are actually quite simple, but are often misunderstood even in grammar books about Spanish.
First, tú, Usted, and vos all mean "you" (singular) in English. By contrast, vosotros means you (plural; sometimes referred jokingly to as "youse" or "you all"). There is no debate about the meaning of each. The confusion lies in when to use them
In Spain, the basic rule is that tú is used with people you are on a first name basis with. You'll use it with friends, family, children and animals, among youth, and even in some prayers (prayers are filled with grammatical exceptions in Spanish, so just memorize as necessary). And young people are tending to use tú over Usted in many situations that older folks would not.
The vos form is not used in Spain except in archaic forms of writing. You can find it in pre-modern literature, or any modern work that wants to give a feeling of being old. Vos comes from the Golden Age of Spanish when it was a polite second-person singular pronoun, and to this day does still appear in some prayers, ritualistic language, and formal occasions. In other words, you won't need it and so don't have to worry about it unless you plan on talking to the King in person.
In Spain, vosotros is always used for the plural of "you", whereas in Latin America it isn't. Instead, Latin American Spanish speakers use Ustedes, which means that many people in Latin America, and learners of Spanish who study Latin American Spanish, cannot even produce the correct verb forms for vosotros. Not a big deal. If you go to Spain, you'll pick them up quickly. If you learned them in class, you'll only need them in Spain.
Finally, vos. It is used primarily in Argentina, though also shows up in Paraguay, Uruguay, most of Central America, and is even heard in the far south of Mexico and at times in Colombia, Venezuela, Ecuador, and Chile. But only in Argentina is it used by everyone all the time.
The formation of the vos pronoun is simple: it's vos, te, tu(s), vos. For instance: ¿Te das cuenta de que estoy hablando de vos y de tu amigo? Remember this example and you'll be able to tell how vos is used in a sentence.
The formation of verbs with the vos form is a bit tricky and varies by region. The basic rule for the present tense is to take the infinitive form, remove the stem and add a stressed vowel plus "s". For instance: tener becomes tenés. But this rule doesn't get you very far, and is often not used anyway.
But don't panic. Argentines understand tú just fine, so don't rush out to master vos if you head there. You won't need it, though it's fun to use. If you want to use vos, listen to how the people around you are using it, then follow their lead.
So if you're learning Spanish, don't get confused about what is ultimately straight forward. The tricky part for most people seems to be getting used to using tú, Usted, and if in Argentina, vos, when appropriate, and of course, conjugating the verb correctly.