I always liked to poke around at things to see how they worked. A while ago I wanted to look inside some of my favorite apps such as the Facebook for Android app:
This is old, however we felt like making a video. So please comment and add helpful suggestions. And there are more ways than one for doing this.
Decompiling *.apks using Dex2Jar and viewing the compiled *.class files in JD-GUI
Dex2Jar - http://code.google.com/p/dex2jar/
JD-GUI - http://java.decompiler.free.fr/?q=jdgui
ApkTool - http://code.google.com/p/android-apktool/
sh dex2jar.sh com.facebook.katana.apk - dumps the compiled java *.class files - creates com.facebook.katana_dex2jar.jar ./jd-gui - finally, run jd-gui and click on com.facebook.katana_dex2jar.jarFor the resources, use apktool
./apktool d com.facebook.katana.apk - this will dump the source files, the xml and images