Unlike English, Japanese has what could be considered “masculine speech” and “feminine speech.” Essentially, it means that men and women speak differently. Normally it isn’t really an issue, however for language learners it can sometimes present funny or awkward situations, for example if you’re a guy and you picked up some Japanese expressions from a girlfriend or female teacher (feminine speech) and you then said them around a group of males while trying to show off your language skills.
Here are some example:
When referring to themselves, men and women can both say watashi (私, わたし), or the more formal watakushi (私, わたくし), but only women would use atashi (あたし) or the more formal atakushi (あたくし). Similarly, only men should use the casual boku (僕, ぼく), or the informal and sometimes vulgar ore (俺, おれ).
There are also many sentence-endings that are gender specific. For example, pretty much any sentence that ends with wa (わ) is feminine. Whereas men will sometimes end a sentence with yo (よ) for emphasis, women will often use wa yo (わよ) or no yo (のよ).
Women will add kashira (かしら) to the end of a sentence to mean “I wonder,” whereas men will say kanaa (かなぁ).
These differences not only occur in everyday speech, but also on TV, in manga (comics), and in video games.