Un, unos, una, and unas are indefinite articles. They have to agree in gender and number to the noun. So if you are saying some girls, it would be "unas niñas" and a male cat would be "un gato".
A helpful chart:
Un- singular masculine
Unos- plural masculine
Una- singular feminine
Unas- plural feminine
There are also definite articles that are used for the girl "la niña" and the male cat "el gato"
El- singular masculine
Los- plural masculine
La- singular feminine
Las-plural feminine
