What is a botnet, and how could a 'DDoS APK' contribute to one?

A botnet is a network of computers or devices that have been infected with malware, allowing a remote attacker (the 'bot herder') to control them without the owners' knowledge. A 'DDoS APK' could contain malware that turns your Android device into a bot within a botnet. The attacker can then use your device, along with thousands of others, to launch DDoS attacks against targeted websites or servers.