What distro are you using? If you put this in your profile, it helps us to help you.
So lets see if Ive got this right, you type a command in, and instead of running whatever program, yelp launches? So typing "ls" launches yelp?
It sounds like you have set some sort of alias. Try moving the files .bashrc, .bash_profile and .profile to .bashrc.bak, .bash_profile.bak and .profile.bak. Log out and log in again, and see if the problem still occurs.