The word ‘us’ is a pronoun, specifically a personal pronoun. It is used to refer to the speaker and at least one other person, functioning as the object of a verb or preposition in a sentence.
For example, in the sentence ‘They gave us a gift,’ ‘us’ is the object of the verb ‘gave’. Additionally, ‘us’ can also be used in prepositional phrases, such as ‘between us’. In this context, it shows the relationship between the speaker and others involved, making ‘us’ an essential part of communication in English.