High level design is the over all architechture means root 
requirement to leaf requirement but low level design's 
consists internal logic requirement of that soft ware.

High level design:
    Overall architecture of the appplication (e.g) data 
flow, flow chart, data structure ect.

Low level design:

   Detailed description  of the each and every module.

HLD - High Level Design 

HLD is nothing but, heirarchy of all possible functionality 
as module in the application.

eg. flograph, flochart, data structure.

LLD - Low Level Design 

LLD is nothing but, static logic bet. every sub module.

eg. ER Diagram, Algorithm

High level Design:  to test the function modules ,operators
etc,.....
Low lwvwl design means to test the 
functions,operatus,submodules   this is called the LLd

Higher level means overall goal of this application like 
application backborn. and lower level design is details 
describe every requriment or every terms. like 
process.decision.document for flowchart.

HLD: DFD , FLOW CHART showing ligh view of app.
LLD : Class diagram with all information like methods etc , 
state diagram

HLD-level Design gives the overall System Design in terms 
of Functional Architecture and Database design.it will show 
system using usecase,class diagram,seqence...deployment 
diagram.
LLD-During the detailed phase, the view of the application 
developed during the high level design is broken down into 
modules and programs.