PPCGeeks - What is in a .cab file?

i extracted a cab on my desktop pc and i see a bunch of files that my pc doesn't recognize... what format are these in?

I noticed when i changed properties to .jpg, its either an icon from the program or a background or something. but i couldn't change it back to what it was cuz i don't know the format name.

this lead me to believe i can change the background image used in the cab to whatever i want, but how? There are a few programs out there that i think i could really customize for my own liking...

This is for my own pleasure, not to redistribute someone else's hard work... ALL RESPECT, DUES AND CREDITS ARE FOR/TO THE DEVELOPER, ALWAYS! i would never take credit for someone else's work, but i WOULD like to customize a little more for myself.

plus, this will help me get into creating my own apps and i have quite an imagination with alot of will power.

can anyone explain this to me?

thanks in advance