what is the difference between usted and tu
Usted is the formal way of saying you. You use this when you are talking to teachers, bosses, strangers, basically anyone that you would show respect to. Usted takes the same conjugation as él and ella.
Usted es una maestra. (You are a teacher.)
Usted es el mejor. (You are the best.)
Tú is the informal way of saying you. You would use this whenever talking to friends or anyone that seems to be younger or the same age as you. You could also use this for someone older than you if they are really close to you. Also, don't get mixed up between tu and tú, they sound the exact same but have different meanings. You would be able to tell which someone is using based on the context. Tu means your, but tú means you.
Tú eres mi hijo. (You are my son.)
Tú eres un estudiante. (You are a student.)
Ella es tu secretaria. (She is your secretary.)
These are only the singular forms of the word you. The plural would be ustedes and vosotros. Ustedes is used more commonly, being used in every Spanish-speaking country. Vosotros is only used in Spain, so unless you want to go to Spain, you don't have to worry about learning vosotros. Ustedes and vosotros are used when you are referring to two or more people. (In Spain, vosotros is used as the informal, and ustedes as the formal. However, in almost every other Spanish-speaking country, ustedes is used as formal AND informal.)
Ustedes/Vosotros son/sois mis padres. (You are my parents.)
Ustedes/Vosotros comen/coméis manzanas. (You (plural) eat apples.)
EDIT: Vos is also used to mean "you." However, this is only used in some Spanish-speaking countries, the most predominant one is Argentina, where tú is almost never used. Other countries that they use vos in is Uruguay, Paraguay, El Salvador, Guatemala, Honduras, Nicaragua, and Costa Rica. Where they never use vos, is Spain, Mexico, Peru, Cuba, Puerto Rico, and Dominican Republic. All the rest of the Spanish-speaking countries use both. The conjugation for vos is actually really simple. If it is an -ar verb, then you take out the -ar and replace is with -ás. If it is an -er verb, then take out the -er and put in -és. If -ir, replace -ir with -ís. The only irregular ones are ser, ir, and haber. The conjugation for them each, in order that I listed them, are sos, vas, and has.
Vos comés una naranja. (You eat an orange.)
Vos dormís todo el día. (You sleep all day.)
Vos tenés todo el tiempo en el mundo. (You have all the time in the world.)
Hope that helped! :)