One important thing to realize about the apps being offered in the App Store is that there is an increasing prevalence of what I call "lure apps", promising great functionality for either very low cost, or free. The old rule of "If it seems to be too good to be true, that is usually the case" certainly applies here. There are far too many companies being unscrupulous, enticing you to download an app which you discover in attempted usage is lacking key functionality until you pay the company/developer some fee (which can even be periodic!) through an in-app purchase. So, watch out for apps with the App Store cite as "Offers in-App Purchases", as these typically extort money from you for functionality which the app description does not mention as missing from the app as downloaded.
Another insidious thing to be aware of is that some iOS apps are not self-contained: they claim high-powered capabilities, but what you don't realize is that they achieve this by uploading your data to a server — where the real power is — and then download and present the result to you as though the app had done it all in your device. This is an obvious security concern: sensitive data (particularly data protected by government regulations and contracts) should not be used in such apps. Unfortunately, detecting such apps when you go to evaluate them is difficult, as the developer typically doesn't disclose this, and reviewers don't look for it. Sometimes the app description may include words to the effect that network connectivity is required, which is the clue to this behavior.