this is a bit concerning
and next time please USE the code tags
wrap the code in them
the code you are "copy / pasting "
this looks like you installed incompatible repos ( a bit hard to do on fedora these days )
please post the output of this
yum repolist all
this will list ALL the repos installed and if they are ON or OFF
also try cleaning the database
yum clean all
Also you might want to change the name on the thread
this looks like you are running "Fedora 17 and not 18 "
it is time to think of "Upgrading"
fedora 17 will hit it's End Of Life in about 40 DAYS