“Tu” is always singular and therefore refers to only one person.
Other words regarding the use of “tu”:
Tutoyer (= to use “tu” with someone, a verb.)
Le tutoiement (= the act of using “tu” with someone. A noun.)
It’s mainly used with equals to signify friendship or a close relationship, or with children or teenagers.
A : Salut Paul ! Tu vas bien ? (Hi Paul! how are you? )
B : Oui, très bien. Et toi, ça va ? (Yeah, I’m fine. How are you doing?)
Note: “Et toi” literally means “And you”.
Other words regarding the use of “vous”:
vouvoyer (= to use “vous” with someone, a verb.)
Le vouvoiement (= the act of using “vous” with someone. A noun.)
It’s mainly used for strangers and/or to show respect for others and/or by children/teenagers towards adults.
With someone you don’t know
A : Pardon Madame ! Savez-vous s’il y a une poste près d’ici ? (Excuse me Madam! Do you know if there is a post office near here?)
B : Désolée Monsieur, je ne suis pas d’ici. (I’m sorry sir, I’m not from here.)
“Vous” can be singular or plural. It can refer to one person or several ones.
Since “tu” is only singular, “vous” should be used when addressing several friends, children or close people.
Switching from “vous” to “tu”
When a feeling of friendship arises, we can start to use “tu” with the other person. One of the two people will often ask: “Est-ce qu’on peut se tutoyer ?” (“Can we use ‘tu’?”); or
say: “On peut se tutoyer, tu sais.” (“We can use ‘tu’, you know.”)