What is UML diagrams:- UML stands for Unified Modeling Language. The UML Class diagram is a graphical notation, which is used to create and visualize object oriented systems. UML describes the structure of a system by showing the class of system's and the relationships between objects. HLD :- High level design. HLD is a overall system design. LLD :- Low level design. LLD also known as a detailed design and it is used to design internals of the individual modules identified during HLD.