Explanation
A buffer overflow (or buffer overrun) occurs when the volume of data exceeds the storage capacity of the memory buffer. As a result, the program attempting to write the data to the buffer overwrites adjacent memory locations.
Buffer overflow is a vulnerability in low level codes of C and C++. An attacker can cause the program to crash, make data corrupt, steal some private information or run his/her own code. It basically means to access any buffer outside of it's alloted memory space. This happens quite frequently in the case of arrays.

Explanation
Diffie Hellman (DH) uses a private-public key pair to establish a shared secret, typically a symmetric key. DH is not a symmetric algorithm - it is an asymmetric algorithm used to establish a shared secret for a symmetric key algorithm.