Onna Kotoba
The Japanese language has some words and some grammatical constructions that are associated with men or boys, while others are associated with women or girls. Such differences are sometimes called "gendered language." In Japanese, speech patterns associated with women are referred to as onna kotoba (女言葉, "women's words") or joseigo (女性語, "women's language").
In general, the words and speech patterns associated with men are seen as rough, vulgar, or abrupt, while those associated with women are considered more polite, more deferential, or "softer". Some linguists consider the rough/soft continuum more accurate than the male/female continuum. For example, Eleanor Harz Jorden in Japanese: The Spoken Language refers to the styles as blunt/gentle, rather than male/female.
There are no gender differences in written Japanese (except in quoted speech), and almost no differences in polite speech (teineigo), since males take on "softer" speech, except for occasional use of wa (and except for the fact that women may be more likely to use polite speech in the first place).
Read more about Onna Kotoba: Traditional Characteristics of Women's Speech, Traditional Characteristics of Japanese Men's Speech, Gender Differences in Modern Society, Problems For Japanese Learners