HLD -- High Level Design (HLD) is the overall system design - covering the system architecture and database design. It describes the relation between various modules and functions of the system. data flow, flow charts and data structures are covered under HLD.
High Level Design gives the overall System Design in terms of Functional Architecture details and Database design. This is very important for the ETL developers to understand the flow of the system with function and database design wise. In this phase the design team, testers and customers are plays a major role. Also it should have projects standards, the functional design documents and the database design document also.
LLD -- Low Level Design (LLD) is like detailing the HLD. It defines the actual logic for each and every component of the system. Class diagrams with all the methods and relation between classes comes under LLD. Programs specs are covered under LLD.
Low - Level Design (LLD) - This document is need to do during the detailed phase, the view of the application developed during the high level design is broken down into separate modules and programs for every program and then documented by program specifications.
HLD refers High Level Design and LLD refers Low Level Design.
Means HLD contains overview of the design and LLD contains detailed design.
HLD: It refers to the functionality to be achieved to meet the client requirement. Precisely speaking it is a diagramatic representation of clients operational systems,staging areas. Also how and what frequency the data is extracted and loaded into the target database.
LLD: It is prepared for every mapping along with unit test plan. It contains the names of source definitions, target definitions, transformations used, column names, data types, business logic written and source to target field matrix, session name, mapping name.