Until yesterday, sms can only be used between phones of the same carrier (free service, i think). Which means i couldn't send an sms to, or receive one from someone who is using softbank or au. Plus, there is a limit of 70 characters when you send a sms (for japanese). half sized characters like english is 160.
But from today onwards, you can send sms to anybody. They made such a noise about it in the news that i almost wanted to roll my eyes. People being concerned about "how scary it is, just by knowing the phone number you can send a message. Spam + unwanted sms from people you didnt want to give your phone email to is likely to increase. blah blah" And there is talk about setting a limit on the number of sms you can send to a particular number, or something. Paranoid japanese people -_-;
This decision took them two years to materialised (Looking into the possibility of using sms between carriers) and what actually caused it to get carried out was the 3.11 earthquake. They realised that with just the phone number, you can contact people easily when the voice network or the email service is down (?). Oh wow. Some serious revelation huh.
Character max limit remains same. Messages between users of the same carriers remain free, but those that are exchanged between different carriers are charged. Receive is free though. docomo, au and Softbank's cost Y3.15 per message while eMobile is Y2.1
An example of sms sent between docomo and softbank. Hurray! For this progress in the history of keitai.