Son is third person plural: ellos son, ellas son, ustedes son. They are, you (plural) are.
Somos is first person plural: nosotros somos. We are.
The infinitive form of the verb is ser.
The other forms would be:
Yo soy. I am.
Tú eres. You (informal singular) are.
And in Spain only, there's a second person plural informal conjugation:
Vosotros sois.