What is it?
Geinimi is a trojan that runs on Android-based phones. The trojan comes delivered as a component of other software and has been found in pirated versions of legitimate applications. Once installed the trojan steals personal information and uploads it to remote servers. How does it work?
Anyone who installs applications from the Android marketplace do not get the compromised version, and Geinimi is not widespread. However, this is a harbinger of things to come. If you don't have an Android-based phone, your only risk is a friend's infected phone coughing up information about you that your friend has. The real message is that smartphones and tablets are being used for commerce now, and this is attracting criminals.
How can I prevent it?
Your first line of defense is to use legal apps and be selective about what you install. There is considerable wisdom in not being one of the first to install a new app.