A Roll-up Summary field is a new field type under System Settings>Objects> Object Fields, designed to summarize or aggregate values from a related list. You can select a field from the related list to aggregate and save the result in a new field on the parent object.
Roll-Up Summary Fields can only be created on:
Custom objects in a lookup relationship to another object
Standard objects on the parent side of a lookup relationship with a custom object
Opportunities using values from related opportunity products
Organizations using values from related opportunities
Note: Roll-up summary fields cannot be created from standard child objects such as Tasks, Events, Notes, Files, or Emails.
Available aggregate calculations for Roll-Up Summary Fields are COUNT, SUM, MIN, and MAX. The field types you can calculate depend on the type of calculation:
SUM: Number and percent fields
MIN or MAX: Number, currency, percent, date, and date/time fields
COUNT: Counts related records matching filter criteria (default selected value)
If the type of aggregate chosen is not COUNT which is the default selected value in the dropdown then the ‘Field to Aggregate’ dropdown is displayed. If MIN or MAX or SUM are selected, then only fields on which you can perform that operation are shown in the dropdown.
Internal system fields, multi-line text fields, image fields, and calculated fields cannot be used in roll-up summaries. Roll-up Summary field calculations occur after a related list record is changed or saved and are processed in near real-time using the appropriate queue.