The Japanese language is unusual among major languages in the high degree to which the speech of women seen collectively differs from that of men. Differences in the ways that girls and boys use language have been detected in children as young as three years old.
Such differences are sometimes called "gendered language." In Japanese, speech patterns peculiar to women are sometimes referred to as onna kotoba (女言葉, "women's words") or joseigo (女性語, "women's language"). The use of "gender" here refers to gender roles, not grammatical gender. A man using feminine speech might be considered effeminate or homosexual, but his utterances would not be considered grammatically incorrect. In general, the words and speech patterns considered masculine are also seen as rough, vulgar, or abrupt, while the feminine words and patterns make a sentence more polite, more deferential, or "softer" (countering abruptness).
There are no gender differences in written Japanese (except in quoted speech), and almost no differences in polite 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).