A somewhat formal or literary term for a person without material wealth. In everyday speech, 貧乏な人 or 貧しい人 are more common.
貧者を助けることは大切な社会の役割だ。
Helping the poor is an important role of society.
He used his fortune to give alms to the poor.
富者 refers to a wealthy person, while 貧者 refers to a poor person, forming a direct contrast in formal/literary style.
Compound of 貧 (hin, 'poverty') and 者 (sha, 'person'). The kanji combination directly conveys 'one who is poor'.