What are the characteristics of a roll-up summary field in Salesforce?

A roll-up summary field calculates values from related records, such as those in a related list. You can create a roll-up summary field to display a value in a master record based on the values of fields in a detail record. The detail record must be related to the master through a master-detail relationship. For example, you want to display the sum of invoice amounts for all related invoice custom object records in an account’s Invoices related list. You can display this total in a custom account field called Total Invoice Amount.
You can perform different types of calculations with a roll-up summary field. You can count the number of detail records related to a master record. Or, you can calculate the sum, minimum value, or maximum value of a field in the detail records.
Before you begin creating roll-up summary fields for your organization, review the implementation tips and best practices.