There is often a plus sign in the upper left corner of the price button for an app. Even some free apps have this plus sign.
Ask Different is a question and answer site for power users of Apple hardware and software. It only takes a minute to sign up.Sign up to join this community
It means that it is a universal app, able to run on the iPad and iPhone.
It means there are in-app purchases available under that app. You can turn that feature off under your General Settings, Restrictions.