Party refers to political power. It is the ability to influence others in terms of rule/law creation. For example, Trade Unions, pressure groups or organised interest groups have political power.
Weber is referring to politics in the broadest sense. We look after our own interests. Summarised, these are our 'market situation'.
Evaluation of party:
Analysis is 'gradational' not 'relational' like Marxism.
Useful as it describes the complex reality of contemporary society using Postmodern ideas like seeing society as 'fragmented'.
Very difficult to measure forms of power as it is subjective.
Not fragmentary as they take pride in their social groupings.
Accounts for the development of middle class, unlike Marx.