Software Tester tests the application from end user 
perspective, so software knowledge is not required.
However, having software knowledge is advantageous but not 
mandatory.

yes software testers need software application knowledge as 
developer has.Becoz testers job is to understand the 
application then only they can fix the bug effectively,wat 
is the areas having bug most,like many issues.If developer 
has to need 8/10 means tester needs 6/10 software knowledge.

Software Engineer requires good knowledge on the application
and general knowledge to do ad-hoc testing.Tester with
development experience is an added advantage.Testing the
application without software knowledge is BLACKBOX Testing
and any person testing w/o s/w knowledge could be called as
Black box Tester.

we are not into whitebox testing we are into black box 
testing so we need not have to know the code we need to 
know how it works .so we need to have an hands on the 
domain and application . its good to have some basic idea 
of code to it will fetch us in the future.