WhatsApp is a multi-platform mobile application that enables a spontaneous chat between users of different smartphones. Some might claim that it can replace SMS, because it enables you to send text messages to others as long as they're on WhatsApp and you have their phone#.
SMS doesn't need to be replaced - it works perfectly as it is. Actually WhatsApp has created a new messaging medium more casual and chat-like than SMS. It's a bit difficult to explain - all I can do really is to recommend WhatsApp to all my friends.
I first heard of this app last year when some UK heads were shouting about it on facebook. And remained sceptical about it for way too long as I didn't realise that it really does work with most smartphones - not just Bold BBs & iPhones:
iPhone. (Requires iOS 3.1 or later) · Android (OS 2.1 or above) · BlackBerry (OS 4.5 or above) · Symbian^3: N8 · C6 · C7 · E7 S60 5th edition: Nokia 5800 XpressMusic · Nokia 5800 Navigation Edition · Nokia 5530 XpressMusic · Nokia 5230 · Nokia 5233 · Nokia N97 · Nokia N97 mini · Nokia X6 · Nokia C6 · Samsung i8910 Omnia HD · Sony Ericsson Satio · Sony Ericsson Vivaz · Sony Ericsson Vivaz Pro · Nokia N8 S60 3rd edition: LG KS10 · LG KT610 · LG KT615 · Nokia 5700 XpressMusic · Nokia 6110 Navigator · Nokia 6120 Classic · Nokia 6121 Classic · Nokia 6124 classic · Nokia 6290 · Nokia E51 · Nokia E63 · Nokia E66 · Nokia E71 · Nokia E90 Communicator · Nokia N76 · Nokia N81 · Nokia N81 8GB · Nokia N82 · Nokia N95 · Nokia N95 8GB · Samsung SGH-G810 · Samsung SGH-i400 · Samsung SGH-i408 · Samsung SGH-i450 · Samsung SGH-i458 · Samsung SGH-i520 · Samsung SGH-i550 · Samsung SGH-i550w · Samsung SGH-i560 · Samsung SGH-i568 · Nokia 5320 XpressMusic · Nokia 5630 XpressMusic · Nokia 5730 XpressMusic · Nokia 6210 Navigator · Nokia 6220 Classic · Nokia 6650 fold · Nokia 6710 Navigator · Nokia 6720 Classic · Nokia 6730 Classic · Nokia 6760 Slide · Nokia 6790 Surge · Nokia C5 · Nokia E52 · Nokia E55 · Nokia E71x · Nokia E72 · Nokia E75 · Nokia E5 · Nokia N78 · Nokia N79 · Nokia N85 · Nokia N86 8MP · Nokia N96 · Samsung GT-i8510 · Samsung GT-I7110 · Samsung SGH-L870
Back in 1960s Marshall McLuhan illustrated how The Medium is The Message. Most of the stuff he wrote&illustrated back then still apply in one form or another. That's why I'm recommending everyone to head over to whatsapp.com and download this app for your device.