Historically, ziemianie consisted of hereditary nobles (szlachta) and landed commoners (kmiecie; Latin: cmethones).
The non-nobles thus had to either sell their estates to the lords or seek a formal ennoblement for themselves (not an easy task), or had their property taken away.
They were the lesser members of the szlachta, contrasting with the much smaller but more powerful group of "magnate" families (sing.
After the Partitions of Poland, at least in the stereotypes of 19th-century nationalist lore, the magnates often made themselves at home in the capitals and courts of the partitioning powers, while the gentry remained on their estates, keeping the national culture alive (for a preserved, non-magnate example, see Dąbrowski Manor in Michałowice).
Although the March Constitution of Poland (1921) abolished the legal class of hereditary nobility, szlachta or ziemiaństwo was informally recognized and remained an economic and social reality as well as a politically influential group, to a degree greater than hereditary nobility in European countries with more highly developed capitalism (and the remnants of feudalism mostly gone).