3 Things You Need To Know About Android SDK

Android uses Dalvik
Dalvik Virtual Machine is a minimalistic implementation of the Java platform. Android apps are written in Java programming language but it is being compiled to a format that Dalvik could understand -- the DEX file, or the Dalvik EXecutable. This DEX format is the output of compiling Java .CLASS files -- not Java source codes.

J2ME won't run on Android
Your J2ME experience will be relevant to writing Android apps, but your J2ME apps won't run on Android. Dalvik VM implements a different set of Java API for mobile -- think of Java and J++ as you think of J2ME and Dalvik.

SDK uses Qemu
Qemu is a virtual machine that emulates a real machine and boots up a guest operating system inside your computer -- in this case, it's the Android OS that gets executed. This basically answers why after pressing Ctrl+F11 on Eclipse, the emulator takes so long to display your running your app and sometimes unresponsive.