Diplomacy is to Risk what the Bridge is to Beggar-My-Neighbour. Diplomacy is a game of international intrigues and power politics where mix:
Strategy: is when and where to attack and who to support or ally; as well as what the relations should be afterwards
Tactics: is how to move your armies and fleets to reflect your strategy
Negotiation/diplomacy: is the communication between players that is the overall framework in which your tactics and strategy are played to achieve your goals.