is this an instant messenger or replacement app for texting

Both.

Use it like an IM, but it links to your phone number and you'll instantly see which of your contacts have it.

It uses the data on your phone to transmit messages. It works over cellular data or over wifi.

I use it for sending pictures more than texting

I also like to use it to send pics as it's free for me, whereas on my tariff there is a charge for sending picture messages.

Its free? how is that different from email?

Doesn't use a user name it just goes by your number. I also use it for sending pictures and messages. I have Kik as well but that uses user names and whatsapp seems to be more popular.

Is there a charge for using it? its just when I downloaded the app it asked for my phone number to send me a text

I think it's free for the first year, then there is small charge (not even a pound) to carry on using it each year.

Its also multi-platform, Android, Blackberry, Nokia, Windows Phone and the other fruit based one.

Can't live without it now, first discovered it when l was working abroad so l could send IMs and Pics for free, rather than use SMS/MMS, when connected to hotel/office wifi.

As other people have stated uses your phones Wifi or Mobile Data connection to access the internet. Will use your mobile phone so it knows who you and can then scan your phones contact list to see who else is on WhatsApp.

Cannot recommend it enough.

Been using it a few years and ive never been charged yet ...

might be due to me flashing ROMs (maybe resetting a counter ?)

Because your username is your phone number, the send you a text to verify that the number you've given is actually your number. Otherwise, it would be fairly trivial to "use" someone else's number and pretend to be them.

I think the charge is for anybody who has signed up in the last couple of months. They may have changed T & C for existing users but I don't think so.

http://www.whatsapp.com/faq/en/general/20965922

Just a quick watsapp question. Is it possible on the iphone to read a message without it appearing as read (2 green ticks) on the persons screen. I'm slightly crafty you see lol, and lazy when replying so don't want to offend friends with a late reply. Thanks

Nope. It's a bit of an annoyance - I always hated people who used SMS delivery or read reports. I'll reply in my own time, thanks!

To clarify: the first tick tells you the message has reached whatsapp's server, the second tick tells you the message has reached the person's phone not that they have read it.

In group messaging, you only get the first tick (reaching server).

Oh right. But is there any way to avoid the 'last seen at 15.30' because that way the other person can see when you were viewing the application.

No way as far as I know (but I'm only familiar with the windows phone version of the app).

I think it's probably unavoidable on other platforms.

Either refuse to use whatsapp or don't be crafty?

From inside Whatsapp:

Settings > Chat Settings > Advanced > Last Seen Timestamp

Great, thanks for that. So just to confirm, this means that your friend can't see the last time that you entered watsapp?