BRD is a high level documentation (means tables) that covers the business aspect of a requirement on a broad level. For example: let’s consider that you want develop a new website. Your BRD would address what does the business want to do. Let’s say it is a website like Amazon.com and it enables individuals to shop on the web. This would be your business requirement covered in the BRS.
FRD is Low level documentation (use case diagrams) that would actually address each function that the website provides in order to make the shopping experience of the people visiting the website easy and effective.
BRD – focus on business needs i.e capturing customer details
FRD – focus on functionality of the system i.e how it is working.