You can create something called ‘Bucket’ field in Salesforce. With Bucket fields you can categorize the records in the report itself and on the fly. You don’t need to create a custom field on the object for this.
For example, let say that if you are creating a report on Opportunity and want to categorize the opportunities in the buckets of ‘small’, ‘medium’ & ‘large’ based on the opportunity amount. You don’t need to define this category as a formula field on the Opportunity object. You can create a bucket field in the report and categorize opportunity records on the fly in report itself.