HLD - High Level Design
Here the design will show the links of the pages from one 
to another

LLD - Low Level Design
Here even the database links are discussed.

for exact definition and difference visit the URL
http://productdevelop.blogspot.com/2007/09/difference-
between-hld-and-lld.html

In high level designing the overall project will be divided 
into modules...........

Where as in low level desigingn the modules are divided 
into sub modules and the acutal coding will be done....

And this is done in the designing phase