KakaoTalk is an application that, like WhatsApp, allows you to keep in touch with all your contacts that have a mobile phone (it won't work with PDA or tablets).
You can send messages, videos and photos to all your contacts, while personalizing chat rooms where you talk to one or a group of friends. All this can be done free, without having to pay per message and by having internet connection via 3G or Wifi.
KakaoTalk is an interesting applicationm, but unfortunately it isn't as advanced as the aforementioned Whatsapp. This has considerably reduced its utility.