Yes, it's free. There used to be a small subscription charge for some users but that was dropped in January of last year. (There are also some hoax news stories going about, suggesting that Whatsapp is to start charging again but they can be ignored).
As already mentioned, it is free to use, but bear in mind it uses your internet allowance to send photos, make phone calls, etc.
However, depending on if you're on PAYG or a monthly contract, each time you use WhatsApp, you'll either use up some of your internet allowance, or you'll pay for it as you use it, so it's not entirely free.