WhatsApp is a messaging service predominantly used on mobile phones.
The main attraction for WhatsApp users is that it addresses two fairly fundamental issues with mobile messaging apps – cost and compatibility.
The two most common mobile phone operating systems are Google Android and Apple iOS. If you use one and a friend uses the other, the only common way, without you can send a message to your friend is via SMS (Text Message) and this is very often not free. Because most mobile phone operators offer internet connectivity across their phone networks, using apps is an easy way to communicate and this is where WhatsApp comes in. It was one of the first messaging services to produce version their app for nearly all mobile phone operating systems. So suddenly Apple iPhone users could send instant messages to Android users without having to pay for an SMS message, effectively for free.
WhatsApp is now owned by Facebook and based in California USA.