Dear friends, just like the vEdAs, upaniShads were not written, they were passed on from guru to SiShya. The ending part of each vEda is known as the vEdAnta and upaniShads are a part of vEdAnta. There are 108 upaniShads totally. Each upaniShad has the name of its preceptor embedded in it. They were taught by RShis of the vEdAs on different aspects of soul. Some upaniShads have the RShis`s names as the title and others have different titles. Example: mANDUkyOpaniShad after RShi maNDUka. talavakArOpaniShad after its original directory, talavakAra brAhmaNa of sAmavEda. Literally upaniShad means the knowledge imparted while sitting near. Its a wonderful world in these scriptures. Any more info needed, please feel free to ask. Thank you.