They

They is the third person plural pronoun in English, used to describe a subject. The object form is "them". "They" can be used for both people or objects. 
'They' can also be used for the singular third person if the gender of the person is unknown, or to purposely be ambiguous, or neither male nor female. Singular 'they' is conjugated the same way as the plural form, similar to the matching conjugations of the plural and singular 'you' in English. Singular 'they' is never used for objects, only people.
'They' can also be used to refer to a generic group of people, such as "In Britain, they celebrate Christmas in December".