WhatsApp is an IM which runs across multiple platforms i.e. Android, BlackBerry, iPhone, Nokia (Symbian). It is same almost same as the BlackBerry Messenger (BBM) but BBM runs only on a BlackBerry and this app runs on all (almost all).
WhatsApp contact is added automatically searching the phone contacts who are using WhatsApp messenger. When the message is delivered single tick is appeared and when the message is read double tick appears in the left side of the message (some what same as BBM).
You can even share images, videos, music, location etc. and you can even know when was your friend seen last in whatsapp.
First year is free for the users and from the second year the rates are as low as $1.99 per year (super cheap). Its a must have app on the phone.