Both were rebel groups created to fight the government. The main difference is geographical: the Viet Minh emerged from the North and fought before the Vietnam War, and the Viet Cong was active in the South and fighting against the United States.
The Viet Minh is known as nationalist, it was formed in opposition against Japanese and French colonial rule in North Vietnam. Its main battle was against the French in order to achieve the country's independence.
The Viet Cong was formed with many of Viet Minh veterans, its ideology was communist. It was formed in opposition to the South Vietnamese government and faded away after Vietnam's unification.