Most malicious or "Rogue" Smartphone Apps are Trojan Apps. A Trojan App can look very professional within the AppStore and once downloaded may well operate as expected and serve the purpose you wanted it for. However once downloaded and used, a Trojan App will perform malicious operations without your knowledge in the background. So the App may well be an entertaining game you play, but as you play the App sends premium rate text messages, suppressing all text message notifications on your phone, so you don't know its happening. The monetisation of the scam is the text messages are going to a premium rate line operated by the fraudsters, costing you £3 each time the App texts. You may not find out until your mobile phone company gets in contact or you clock very high text message costs on your bill. Of course by this time the bad guys will have cashed out and closed the text line.
Given the 100,000s Apps in AppStores today, and the 1,000s of new Apps which are released every week, there is always the potential new rogue Apps could slip through any of these smartphone heavyweights AppStore security nets, therefore user vigilance is necessary.