According to WhatsApp, it will be tracking user metrics to check on things like how often people use their services to “better fight spam on WhatsApp”. Your number will also be linked to Facebook, so when you use Facebook you will see friend suggestions of people you know and more targeted ads.
WhatsApp reiterated that WhatsApp itself will not show ads, that your conversations are end to end encrypted meaning only you and the person you are talking to is privy to it and not even WhatsApp and Facebook, and that your number will not be handed over to advertisers.
For now, these changes are opt-in, meaning you will be asked to share your account info, and you can choose not to. The option will appear as a popup to users once it starts rolling out.
Apart from this no other change has been announced for the product.