Emojis is the Japanese term for emoticons. Every mobile phone in the Japanese area has a fixed set of emoticons included and they are all compatible between different manufacturers of mobile phones.
Apple chose to include them as well, but didn't use the defacto-standard encoding for them but decided to encode them in the private use area. So the emoticons are only compatible between iPhones. Read more about it at inner.geek.nz.
To use the Emojis, the iPhone has a special keyboard. But because of the incompatibility, the keyboard is only available for SoftBank Mobile customers. But there exist some programs in the AppStore to enable the keyboard regardless of the SIM card.
But don't get ripped off. You don't have to pay for e.g. iEmoji - which is totally useless after the keyboard has been enabled.
Just get Spell Number for free, start it and enter the
91929394.59. After that, go to Settings → General → Keyboard → International → Japanese
and enable the Emoji keyboard. Done.
You can find these instructions with pictures at maketecheasier.com.