What is Roll Up summary field in Salesforce?

We can perform different types of calculations with your roll-up summary fields. You can count the number of detail records related to a master record, or calculate the sum, minimum value, or maximum value of a field in the detail records.

Note : Roll up summary field can only be defined on the master object.

While your formula fields calculate values using fields within a single record, roll-up summary fields calculate values from a set of related records, such as those in a related list.
The Roll up Summary field is basically of 4 types:
Count
Sum
Min
Max