Before Learning Android :

Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.

To learn Android,basically you should know core java concepts like :

1)Classes,Objects.
2)Overiding,Overloading,oops concepts.
3)"Is-A" relationship ,"Has-A" relationship.
4)Inheritance
5)ExceptionHandling.
6)Pojo classes.
7)Static variables,methods,classes.
8)Collections(Arrays,Arraylist,vector,Maps etc).
9)JDBC connection.
10)Atleast one working exp on java will make ur stuff easy to learn android.