Ignore the first answer you received on here...
Japanese phones started out with half-width character text messages - a carry over from the pager days. Your phone had to be on to receive the message, as it was sent directly to the phone number. This died out pretty quickly, and was replaced with email-like systems. This also hit a dead end, and now all Japanese phones use regular email. There is no limit on the number of characters or anything like that.
In other words, it`s much more free than SMS.
For some weird reason, people overseas seem to assume that if SMS isn`t used, that you can`t send text in Japan. This is hilarious, as people send countless numbers of "mail" to each other. It`s far far more common than actually calling someone.
Just like SMS overseas.
To be compatible with the international market, phones also support SMS - but no one uses it as far as I can tell. There is no advantage to it, really. Character numbers are limited, and it actually costs more than just emailing someone from my phone (Which is technically free for most people as almost everyone has unlimited data plans). When people exchange cell numbers, it`s usually done by IR between the phones - and includes their cell email info.