Google's Android operating system routinely gets negative press for the thousands of malicious apps you can download. However, most of those apps are only found in third-party app stores. In general, as long as you stick to apps in Google Play, everything is OK.
We have to say "in general," because despite Google's security there is a type of malicious app that keeps sneaking into the store. In fact, Google just pulled 13 more apps of this type that were up for a months before they were discovered.
The apps in question carry the Brain Test family of malware. If one of these gets on your phone or tablet, it takes full control. From there, it can download other malicious apps from third parties. The app will also sneakily download other apps containing Brain Test from the Google Store, which boosts those app's numbers and make them look more trustworthy.
At this time, Brain Test apps don't steal personal information, the hackers behind it make money by guaranteeing they can sneak a client's app onto gadgets. However, it could easily be used to steal information in the future, so you definitely don't want it on your gadget.