fordern = to demand
erfordern = being neccessary,
so there IS a difference ;-)

fordern - demand, claim, postulate
erfordern - require, necessitate

"fordern" - the subject can be a person or a circumstance.
"erfordern" - the subject is always a person.
I think...

Basically the difference is as follows:
"fordern" is used for persons, institutions, and laws (implying some kind of "authority"), whereas "erfordern" is used in more "abstract" contexts (for situations, circumstances etc.) ...
Well, so much for now - please feel free to ask again if you are still confused and/or need more detailed information!

Die Lage erfordert sofortige Maßnahmen.
Die Opposition fordert sofortige Maßnahmen.

cuwak
It's the other way round.

Thanks everybody for the clarifications.