Rollup fields store calculations on fields from 1:n relationships. For example, the sum of estimated sales for an account from open opportunities or the number of cases linked to an account.
To create a rollup field, in a solution, add a new field to the appropriate entity and select Rollup as the field type.
Then click Edit and in the Rollup Field editor, select the related entity, optional filter (for example, only open opportunities) and aggreation type and field.
A system job that performs the calculation is created. However, a user can recalculate the value of a rollup field at any time within the form for a specific record. The field is shown on the form as read-only with a small calculator icon to show that it is a calculated field. When hovering over the calculator icon, a tooltip shows when the field was last updated.
Only the following field data and format types can be used in rollups:
- Whole Number / None
- Whole Number / Duration
- Decimal Number (but not Floating Point Number)
- Date and Time / Date and Time
- Date and Time / Date Only
There is more information in this Technet article.
This video from Microsoft provides an overview of rollup and calculated fields.