Being able to download apps for your smartphone or tablet is a brilliant feature and is what makes the modern portable device so versatile. However not all apps are free and it can sometimes be confusing as to what you are paying and how many times you have to pay for an app.
Although in essence there are only two types of apps, free and paid for, the reality is far different. Companies use a number of techniques to try and get customers to pay for their apps.
Sometimes apps are offered as free trials, which means you can use the app for a certain amount of time; but for example, after a month or so you then have to purchase the full app to continue using it.
Some apps are often available for free in a 'lite/light' format, this means developers have restricted access to certain features in the app, that can only be accessed once you have paid for the full app. This is where you are most likely to find in-app purchases.
Another common place where you can expect to find in-app billing is with publishing companies like Zinio or Amazon Kindle, where you can download the app for free, but if you wanted to read a magazine or book you then have to purchase them.
There is no need to worry unduly though, if you haven't jailbroken a iOS device or downloaded a unverified app on your smartphone/tablet, then you don't have much to worry about with in-app billing. The apps will use the same billing process as they use from the App store or the Android Market and therefore will use the same debit/credit card too.
However, you can never be too safe when it comes to online and mobile purchases, so have a read of these: