What is HLD?

High Level Design gives the overall System Design in terms of Functional Architecture and Database design. It is very important document of any software and very useful for the developers to understand the flow of the system. It gives the complete overview of the system and tells to everyone how the system is going to divided into sub systems (functions, modules). In this phase design team, review team (testers) and customers plays a major role. For this the entry criteria are the requirement document that is SRS. And the exit criteria will be HLD, projects standards, the functional design documents, and the database design document. Further, High level deign gives the overview of the development of product. In other words how the program is going to be divided into functions, modules, subdivision etc.

What is LLD?

Low Level Design (LLD): During the detailed phase, the view of the application developed during the high level design is broken down into modules and programs. Logic design is done for every program and then documented as program specifications. For every program, a unit test plan is created. The entry criteria for this will be the HLD document. And the exit criteria will the program specification and unit test plan (LLD).
The Low Level Design Document gives the design of the actual program code which is designed based on the High Level Design Document. It defines Internal logic of corresponding submodule designers are preparing and mapping individual LLD's to Every module. A good Low Level Design Document developed will make the program very easy to be developed by developers because if proper analysis is made and the Low Level Design Document is prepared then the code can be developed by developers directly from Low Level Design Document with minimal effort of debugging and testing.