low and high level test cases are related to low and high level 
bugs....if bug level is low or if any where we think there 
might be low level bug occr tht time we make a low level bug 
and the time of the high level bug we make the high level bug 
test cases.

High level Test cases are those which covers major 
functionality in the application (i.e retrive, update 
display ,cancel (functionality related testcases) ,database 
testcases ).
Low level test cases are those which are related to UI 
related testcases.

High level test cases are those where we test the 
individual functionality related test cases according to 
the requirements but in low level testing we test the 
granules(micro) level of the test case like what is 
expected and is actual result.

Low Level Test Cases:
Test cases for 1.Unit testing 
               2.Integration testing 
are called as Low level test cases.

High Level Test Cases:
Test cases for 1. Functional testing
               2. System Testing
               3. Acceptance Testing 
are called as High level test cases.

High Level Test cases cover the whole application in a 
broader way. They dont cover the functionalities in detail 
but the overall application should work fine. whereas, Low 
level test cases cover each andevery individualunits of 
code and test cases are generated to test each and every 
unit in depth. Low level testing is more detailed then High 
level testing.

White box testing is called Low level Testing
Black box testing is called High Level Testing