WhatsApp is very popular messenger among Android and iOS users and you could use this app for free for the first year. When this period has ended then you had to pay very small mount ($1 or £1, depending on market) for the next year. Now, it is complete free!
Why did they decide to remove this small charge? The reason is very simple. They want to attract as many users as possible and a lot of people do not have their own credit card. Therefore they could not pay this small fee, even if they wanted. Now this problem disappeared. So, we have additional great multi platform (iOS, Android and Windows Phone are supported) free applications, which provides text and voice communication. Soon, also video calls will be available. But, it is not known when.