OMB Circular A-21, Section D.1, states:

A cost is considered direct when a specific grant or contract gains explicit benefit from the cost for a specific programmatic purpose. For example, a PIs salary, when represented by scientific effort on a particular grant or contract, is an example of an allowable direct cost to that grant or contract.
OMB Circular A-21, Section E.1, states:
At educational institutions such costs normally are classified under the following indirect categories: depreciation and use allowances, general administration and general expenses, sponsored projects administration expenses, operation and maintenance expenses, library expenses, departmental administration expenses, and student administration and services.
Indirect costs are infrastructure costs of the University that support the programs of the institution, including research and other sponsored programs. Examples of indirect costs are building depreciation, maintenance costs, the cost of electricity and heat, accounting services, personnel services, departmental administration, purchasing and human subjects administration.
Refer to GIM 22, Indirect Costs: An Explanation, for additional information regarding indirect costs.