Like people and other objects, hours records can have custom fields associated with them. Any custom field that's associated with hours records will appear when adding or editing hours records, and in all reports based on hours.
See Also
Crediting Volunteer Hours
Crediting Volunteer Hours from Assignments
Crediting Volunteer Hours - Simplified
Crediting Volunteer Hours Individually
Crediting Volunteer Hours for Multiple People at Once
Hours/Accounts/Outcomes Integration
View and Edit Hours for Many Volunteers at Once