Gross Operating Income (also called Effective Gross Income) is the potential rental income less any vacancy and credit losses. This is the amount of rental income that the owner can reasonably and realistically expect to collect after accounting for property vacancies, tenant concessions and other income opportunities.
Operating expenses include all expenditures required to operate the property including real estate and personal property taxes, property insurance, management fees, repairs and maintenance, utilities, and other miscellaneous expenses (accounting, legal, etc.).
The Net Operating Income is the amount of income a property can generate after deducting Operating Expenses. The Net Operating Income is a strong indication of a properties revenue generating potential.