Unhappy [Q] [HELP]Home.apk and framework.jar

Hey Guys, i want to know a few things.
I want to know what's the importance of framework.jar
What is the relation between Home.apk and Framework.jar
Why always when someone port a home app he includes a framework.jar file with it ?
Hope you answer me and Thanks
My Devices:
*Sony Xperia J (ST26i) Unlocked Bootloader & with Doomsday ROM with vengeance kernel OVERCLOCK TO 1.3GHz
My works:
Doomsday ROM for Xperia J
Illusion ROM for Xperia J
How to create your own stock based XPERIA ROM
Devices i develop for:
Sony XPERIA J
Sony XPERIA E
Sony XPERIA Miro
Samsung Galaxy Core