Pluralism and elitism are 2 different theories that are used to explain how power is spread between individuals and groups in society.
Pluralism suggests that power is spread amongst different groups in society, and that it is the pushing and pulling between these different groups that explains how decisions are made and how governments are influenced. powerful pressure/interest groups are a classic example of pluralism because they show how power is divided amongst these different competing groups and not concentrated with one organisation.
Elitism is the exact opposite of pluralism. In elitist theory power is seen as concentrated amongst a few groups or individuals, including the government. To use the same example as before; elitist theory would suggest that only a few major pressure/interest groups, such as the NRA or AARP, actually have any influence, and that even they struggle to gain power because it is concentrated in the hands of a few members of government.
Of course both these theories have evidence to support them, and most theories use elements of these umbrella theories to explain how power is divided.