Ser is one of two Spanish verbs that convey the sense of the English word "to be".  The other word is Estar. 

yo soy I am
tú eres You are (informal)
usted es You are (formal)
el es He is
ella es She is
Nosotros somos we are
Nosotras somos we are (only girls)
ustedes son you are (plural)
ellos son they are
ellas son they are (only girls)

Note that you have to use different pronouns for different situation.

A group of people one of which you are

1) If you there is a only boys or both boys are girls you say - nosotros somos
2) If there are only girls in the group you say - nosotras somos

A group of people not including yourself who you are referring to

1) If there is only boys or both boys and girls you say - ellos son
2) If there are only girls you say - ellas son

These are some of the uses of the verb "Ser".  You don't always have to use the pronoun before the verb.

To identify the subject by linking it to a predicate noun or pronoun

Ella es ingeniera. She is an engineer.
Ese animal es vaca. This animal is a cow.
To indicate origin
¿De dónde eres? Where are you from?
Soy de Montville. I am from Montville.
To indicate what something is made of
La alfombra es de lana y las cortinas son de algodón. The carpet is made of wool, and the curtains are made of cotton.
To indicate possession/ownership
La mochila es de Miguel. It is Miguel's backpack.
¿De quién es el estéreo? Whose stereo is it?
To indicate destination or recipient with para (for)
¿Para quién son las flores? Who are the flowers for?
Son para mi mama. They are for my mother.
To indicate time of day or other time expression
Es la una y media. It is one thirty.
Ya es tarde. It is already late.
To form impersonal expresssions
Es importante comer bien. It is important to eat well.
To indicate when or where something takes place
El concierto es a las ocho. The concert is at eight.
La junta es en el auditorio. The meeting is in the auditorium.
With adjectives to describe qualities or characteristics that are permanent or basic to the individual or object described
Es una casa muy grande. It is a large house.
Las fresas son rojas. Strawberries are red.
El hielo es frío. Ice is cold.