This second-person pronoun is used to directly address a person or a group of people. In many languages, including Spanish and French, there is a polite and informal form of you to be used in different contexts, however in English, this distinction does not exist.
At the turn of the seventeenth century, thee and thou reached an all-time high in written usage. However, by the 1620s, you had overtaken these now old-fashioned-sounding words.
In English there used to be both plural and singular forms of you. Thou (nominative) and thee (objective) were used to refer to an individual, while ye (nominative) and you (objective) were used to refer to a group of people or as a polite address. The nominative plural form ye gradually transitioned into you, making ye uncommon by the seventeenth century. The singular forms took on informal connotations, and fell out of usage in favor of the polite you. By the mid-1600s, you was used in the context that thee, thou, and ye had historically been favored.
In American English the pronoun you has been supplemented by additional forms to make clear the distinction between singular and plural. You-all, often pronounced as one syllable, is a widespread spoken form in the South Midland and Southern United States. Its possessive is often you-all's rather than your. You-uns (from you + ones) is a South Midland form most often found in uneducated speech; it is being replaced by you-all. Youse (you + the plural -s ending of nouns), probably of Irish-American origin, is most common in the North, especially in urban centers like Boston, New York, and Chicago. It is rare in educated speech. You guys is a common informal expression among younger speakers; it can include persons of both sexes or even a group of women only. See also me.