I use it all the time and there was a guy on the radio on about security and hacking the other week saying the messages are encrypted so no-one involved in the running of whatsapp or even the security services can access the information.
Their claims are somewhat misleading though:
"Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends."
The messages you send come out of your data allowance. If you are on PAYG or have a fairly low cap on your data usage, they could end up costing you considerably more than an SMS message - especially if you are sending/receiving lots of images, Emojis, and the like.
An app is a programme you can download and there are hundreds of thousands if not millions to choose from. There are loads that are free and some that cost a few pounds max. If you look on your 'phone or tablet there will be an apps store where you can search in various categories or type in a word for what you are looking for. I downloaded a flashlight app for free and it has been a godsend no end of times.
TheChair has really said all you need to know .... it's an app that allows you to send texts and picture messages, using your mobile phone, via the internet.
If you've plenty of internet data in your monthly allowance, then this is great .... however, you've got to be careful of going over your internet allowance, or if you pay for your internet as you use it, as this could make it cost more than using the traditional method of sending your messages.
The messages don't come out of your text messaging allowance but as it uses the internet to send messages/images, it will come out of your data allowance.
If you use it when you are connected to WIFI, you won't be using any of your allowance. I use it all the time, as do my family, we have various groups and is great to send photos,videos, and messages.
I suggest that all folk who are using, or others who are thinking of using Whatsapp, should Browse the Internet and read for themselves that Whatsapp is Far from Safe. All of one's Contact Lists are passed to Whatsit's Server and are then available for even Amateur Hackers to access them and read everything which has been forwarded to all other Whatsappers (if that's the word) and even messages which are Not intended for Internet publication of Non Whatsappers who are listed as contacts.