Tú eres una mujer. - You are a woman. Tú eres una niña. - You are a girl.
So what is the difference between the two phrases that translate to 'You are a girl.'? The difference is 'Usted es' is the formal and 'Tú eres' is the informal version of 'you are'. 'Es' is usually used when we are talking about a third party to the conversation like a he, she or an it. 'Eres' is used when we are talking about the person we are talking to. Using different versions of a verb depending on who we are talking about is called verb conjugation. In formal speaking we use the word 'Usted' for you and the verb that goes with it is the same verb we would use with he, she or it.