SRS - System Requirements Specification document
FRS - Functional Requirement Specification document
BRS - Business Requirement Specification document
In other words, SRS = BRS+FRS
These are the formal documents that would be prepared by the Business Analysts (BA's) once the requirements are frozen by the client. Client would specify his system requirements related to the Functionality/Business in the form of checklist meetings/discussions etc . It's BA's responsibility to attend these meetings and prepare the BRS/FRS/SRS. Finally these will be reviewd by the client and would get approved. BA's would document these requirements formally and circulate them in the version wise to the required team to start with.
SRS is also referred as URS - User Requirement Specifications.
The terminology will differ from company to company based on their usage. More or less they mean the same as specified above.