"Android software development is the process by which new applications are created for the Android operating system. Applications are usually developed in the Java programming language using the Android Software Development Kit, but other development tools are available. As of October 2012, more than 700,000 applications have been developed for Android, with over 25 billion downloads. A June 2011 research indicated that over 67% of mobile developers used the platform, at the time of publication. In Q2 2012; around 105 million units of Android smartphones were shipped which acquires a total share of 68% in overall smartphones sale till Q2 2012."
In addition to Java, you also will need to know XML for basic applications. XML is about as easy as it gets, although it is technically a mark-up and not a true programming language, so if you are already comfortable with Java, it should be no problem to pick it up.