Malware, short for "malicious software", is an umbrella term for software (apps) that is hostile, intrusive, and/or annoying.
The following are considered malware:
- trojan horses
Due to the way Android works, the classic mechanisms for malware propagation do not work; Android apps are walled off from each other and the underlying storage and can only interact with each other in very specific ways.
This is not to say that there have not been any malicious apps. Fortunately, they have been very few and, by and large, distributed through alternative app markets and not through the Google Play Store. (There are, however, a few documented cases of malicious apps in the official Google marketplace, but Google pulled them from the site and from devices that had installed them.)
For more information, visit the Malware article on Wikipedia.
If your question is about issues with an anti-malware app, please use the antivirus tag instead.
- The ultimate Android malware guide: What it does, where it came from, and how to protect your phone or tablet
- How to figure out which app is "doing X" (e.g. sending SMS, placing ads) is described here and here.