Basically the same as a computer. If you avoid installing random untrusted apps, you are generally safe (i.e. don't install random no-name Candy Crush clones from the Play Store every week like some people in my family like to do).

Every once in a while there's a more serious vulnerability that can be exploited remotely like Stagefright, but those are fairly rare and if you're here, you will probably hear about them.