What are the major roles of an administrator in a company ?

The roles of administrator are depend on each department of a company (finance, general affair, legal, hr and etc). However, the general role of an administrator is to keep a company proceed/ running.

organisation is the most important work of an administrator , 

basically the work of administrator is lyk guardian to run the organization and take care of all activities necesary to acheive common goal

After planning and organizing, the administrator must establish the direction that others in the organization will follow. Leading the individuals within the organization to accomplish a common set of goals requires the combination of resources and an effective support system. An administrator should have interpersonal skills that can manage the many different personalities that exist within the organization. Additionally, an effective administrator will use the capabilities of the staff to make the plan succeed. This includes the delegation of authority, responsibility and control to other supervisory staff.

To see that Everything is in Order and to make necessary deceisions to keep every mechanism properly function,  under all reasonable conditions/circumstances. He should be an expert Quick Deceision Maker.