Uninstalling a software
 

i installed some rpm packages like xfce4 and i resolved to remove them off my system. How do i do this? What command will it be?

rpm -e <package_name>

If you aren't sure how the package itself is named in the system then first make a query

rpm -qa | grep <name_you_think_it_is>

example for the query:
rpm -qa | grep kernel
This will list all rpms where the name includes the word "kernel".