What does CWM back up? I made a backup using ClockworkMod Recovery v6.0.4.7d of my Samsung I9305 (it runs a rooted stock Android 4.4.4). I selected "Backup and restore" and then "Backup to /storage/sdcard1". The resulting image was of about 7GB.

Does this backup contain:


The firmware?
My modifications to the firmware such as root, Xposed Framework, installed Xposed modules, etc.?
My installed apps?
App data?
Other data I had stored on the internal SD card?


And finally, is compression utilized?
According to: https://wiki.cyanogenmod.org/w/ClockWorkMod_Instructions#Making_a_backup


  ClockworkMod's backup/restore feature will generally only back up the
  /system, /data, /cache, /sd-ext, and /dalvik-cache areas. Other
  partitions such as /emmc, /sdcard, and /media won't be backed up. 
  
  In
  fact, the backed up system will typically be saved to the /sdcard/
  partition in the /clockworkmod/backup/ directory, which, depending on
  your device, may or may not be actually located on a physical
  removable SD card.


According to your question:


  
  The firmware?
  


Definitely Yes.


  
  My modifications to the firmware such as root, Xposed Framwork, installed Xposed modules, etc?
  


Definitely Yes.


  
  My installed apps?
  


Definitely Yes.


  
  App data?
  


Definitely Yes.


  
  Other data I had stored on the internal SD card?
  


Definitely NO, /sdcard is not backed up, because of there would be a CWM backup. Read: http://forum.xda-developers.com/galaxy-s3/help/how-to-backup-internal-sdcard-t1999853

Hope it help