What's a quintile and How does it work?

In Chile The social-economical conditios of families is measured by quintiles (5) and calculated the following way:

<<< Earnings of all the family / amount of members of such family = Per-Cápita Fin >>>

Quintile I: Families whose Per-Cápita is monthly US$110,80 or over

Quintile II: Families whose Per-Cápita is monthly US$187,64 or over

Quintile III: Families whose Per-Cápita is monthly US$293,05 or over

Quintile IV: Families whose Per-Cápita is monthly US$530,47 or over

Quintile V: Families whose Per-Cápita is over monthly US$530,47