Revenue expenditures are those that will provide economic benefits only for the period when the expenditure was made.

Examples:

Rent of office space; Selling costs, salaries, interest payments

Capital expenditures are those that will provide economic benefits for more than one period.

Examples:

Acquistion of land, building, and other equipment, goodwill, or long-term investments

The tricky part I think is when it comes tor epairs and maintenance. The nature of the expenditure should be analyzed whether it extends the life of the asset or increases its quality long-term. In those cases, it should be classified as asset.

Try to browse at intrmediate accounting books. It is explained in more detail there.