LINE is a program of that special breed of immigrants from mobile to PC. It's an application originating from smartphones, its purpose being communication.
Basically, LINE is a messaging client in the vein of WhatsApp or Skype. Its simple interface that's typically minimalist (this was a mobile app, after all) offers various options such as text messages, voice calls, video calls, etc. All features are pretty standard, but very handy in a freeware product. The nice thing about LINE is that it is cross-platform by default, letting users communicate despite having different operating systems. The program works with Android, iOS, Windows and Mac OS X.
Settings are moderately extensive, with the option to adjust notification, chat view and functionality, edit your profile, manage friends list, etc. There are options for blocking unwanted users, attach and send files or images. Text messaging is aided by a library of expressive emoticons, much more complex and detailed that your standard set of smiley faces.
All in all, LINE is a good tool for casual communication. It's stable, fast, lightweight and functional. If you don't like bland classic messaging apps, it's worth trying out.