Diffie-Hellman understanding

In [DH76] Diffie and Hellman describe a means for two parties to
   agree upon a shared secret in such a way that the secret will be
   unavailable to eavesdroppers. This secret may then be converted into
   cryptographic keying material for other (symmetric) algorithms.  A
   large number of minor variants of this process exist. This document
   describes one such variant, based on the ANSI X9.42 specification. 
from rfc2631 

Diffie-Hellman is a key exchange algorithm, it is a method of creating a key, not an encryption method, so the key must be the same as one of the other Encryption algorithms are used in combination. The purpose of this key exchange technology is to enable two users to safely exchange a key and encrypt the subsequent messages.