What are rollup summary fields and what are their limitations?

Rollup Summary fields aggregate values from child records in a master-detail relationship up to the master record. Supported aggregate functions:
— COUNT: Number of child records matching the filter criteria.
— SUM: Sum of a numeric field across child records.
— MIN/MAX: Minimum or maximum value of a date, number, or currency field.
Limitations:
— Only available on the master object in a master-detail relationship (not lookup).
— Maximum 25 rollup summary fields per object.
— Cannot reference formula fields in the rollup calculation.
— Rollup summary recalculations can be resource-intensive on large datasets. Mass data operations that update many detail records may need to be broken into batches to avoid triggering excessive recalculations.
— Cannot be used in custom report types as a grouping field without limitations.
Alternative for lookups: Use cross-object formula for single-record reference (no aggregation), or Apex/Flow for aggregated rollups on lookup relationships.