In fact,
  Vosotros is the plural for tú
while Ustedes is for usted.

https://answers.yahoo.com/question/index;_ylt=A0LEVr8M4cVUF30AGwwnnIlQ?qid=20090316071919AAYlRYE

In Butterfly Spanish, the woman told me "Ustedes is for both tú and usted".
Today I found it's wrong.

Yo soy   I am  
Tú eres   You are (familiar)  
Él es   He is  
Ella es   She is  
Nosotros somos   We are  
Vosotros sois   You are  
Ellos son   They are  

ellos,ellas,ustedes      son



Although "Vosotros" and "Ustedes" can both mean "you" (plural), they're followed by different conjugated verbs.

For example, for the verb "ser",
Vosotros sois
Ustedes son

Tú eres
Usted es

In other words, "usted" and "ustedes" should be with the conjugated form of the THRID person. It should be regarded as the THIRD person, not the second.....

For more details of the Spanish verb "ser", click :

Chinese version:
http://tieba.baidu.com/p/717897281

English version:
http://irregularspanishverbs.com/ser/

Ser
 
The irregular verb ‘ser‘, meaning ‘to be’, is the most frequently used verb in the Spanish language.
It is a confusing verb to say the least as it is one of two verbs in Spanish that mean ‘to be’, the other being ‘estar’.
In English, both ‘ser’ and ‘estar’ may well have the same meaning but each of them are very distinctive verbs but even though this is the case it can be very hard to explain one without making reference to the other and for newcomers to the Spanish language, the confusion about the choice of using ‘ser’ or ‘estar’ is almost inevitable to begin with!
Here we have a basic conjugation of the verb ‘ser’ in the present tense only.
(For a complete and printable conjugation sheet, click on the image on the left or here)

Yo soy   I am  
Tú eres   You are (familiar)  
Él es   He is  
Ella es   She is  
Nosotros somos   We are  
Vosotros sois   You are  
Ellos son   They are  
 ‘Ser’ is used to explain the following: 1. Identity: If you are announcing yourself you use ser.

Yo soy Juan   I am John  
Nosotros somos la familia de Martinez   We are the Martinez family  
2. Occupation:

Yo soy un estudiante   I am a student  
Tú eres médico   You are a doctor  
3. Characteristics or Quality:

El niño es alto   The boy is tall  
Nosotros somos inteligentes   We are intelligent  
4. Origin:

Nosotros somos de España   We are from Spain  
Vosotros sois mejicanos   You are Mexican  
5. Possession:

Los libros son míos   The books are mine  
6. Time, Date or Day:

¿Qué hora es? Son las cuatro   What time is it? It is four o’clock  
Hoy es miércoles   Today is Wednesday  
7. Religious, Political or ideological affiliation:

Ella es hindú   She is Hindu  
Ellas son comunistas   They(feminine)are communists  
8. Location of an event:

El concierto es en la playa   The concert is on the beach  
La fiesta será en mi casa   The party will be at my house  
9. Impersonal expressions:

Es importante   It is important  
Es mi elección   It’s my choice  
Fue difícil pero necesario   It was difficult but necessary  
Es sorprendente que no puedas hacerlo   It is surprising that you can’t do it  
10. The material that something is made from:

La casa es de madera   The house is made of wood  
11. Relationships:

Ana es mi madre   Ana is my mother  
12. Passiveness: The use of past participles to form the passive voice is much less common in Spanish than it is in English.

La canción fue oída   The song was heard  
Son usados para comer   They are used for eating  
El governador fue arrestado en su propia casa   The governor was arrested in his own home  
Other posts and pages about Ser that you may find interesting: How to Conjugate Ser in the Preterite Tense

La fiesta means the party, in English

In my dictionary, in the section of Conjugation of Spanish Verbs on page xi, there's:

77
ser: IND soy, eres, es, somos, sois, son; SUBJ sea, seas, sea, seamos, seáis, sean; PRET fui, fuiste, fue, fuimos, fuisteis, fueron; IMPF era, eras, era, éramos, erais, eran; IMPER sé; PRP siendo; PP sido

soy, eres, es are for the first, second, third singular and somos, sois, son are for the first, 2nd, 3rd plural, respectively.
It should be kept in mind that "Usted" and "Ustedes" should be regarded as the THIRD person, so they are followed by es and son, respectively, instead of eres and sois.