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.