why wont my headset work on the back of my pc

Headset as in headphones? If so, user error in the audio settings.

If you mean the headset has no mic support when connected to the line-in in the back of your computer, that's probably because you don't have a dynamic mic and need a powered mic line like the one in the front of the computer.
 
if your mb has realtech audio drivers make sure you have the newest drivers and in the audio control panel there a check box that make the front and rear audio two ports not one. also make sure under windows sound device realteck speakers are set as default.
 


headphone and speaker jacks are the same. as for USB, what for? doesn't seem like the USB is used for audio. If you have two cables, one for the headphones and one for mic, and your motherboard has a "line in" instead of "mic" port in the back, that's probably the issue right there.