A WhatsApp FAQ explains how these notifications work. Check marks appear next to each message you send, one gray check mark means 'message successfully sent', two gray check marks mean 'message successfully delivered to the recipient's phone', and two blue check marks mean 'the recipient has read your message'.
In a group chat, the second check marks will appear when all participants in the group have received your message. The two blue check marks will appear when all participants in the group have read your message.
In additional WhatsApp news, 9to5Mac has spotted an article from droidapp.nl that leaks calling icons from the WhatsApp Android app. It appears as though the app will gain some type of VOIP calling feature in the future.
Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.
You can download WhatsApp Messenger from the App Store for free.
Read More [Download] [via DroidApp] [via 9to5Mac]