debug just do debugging the application
while release do debugging as well as release the application

- The biggest difference between these is that: 
	  In a debug build the complete symbolic debug 
information is emitted to help while debugging applications 
and also the code optimization is not taken into account. 
While in release build the symbolic debug info is not 
emitted and the code execution is optimized. Also, because 
the symbol info is not emitted in a release build, the size 
of the final executable is lesser than a debug executable.

	- Usually they are configured to build 
differently.  The debug build includes building debug 
symbols (the .pdb files that live next to the .dll 
assemblies) and doesn't perform code optimization which 
speeds up compiling a bit.

Here is a good article which explains clearly the difference
between RELEASE and DEBUG Builds:
http://dotnetthoughts.wordpress.com/2007/11/29/debug-and-release-builds/

Below given is the conclusion of above link:
1. 
We all know that there is no .PDB file generated in RELEASE
database.
2. 
If we see the IL(Intermediate Lang) code using ILDASM then
we find that DEBUG build code has some extra variables to
store the current values of variables and some extra nop(no
operation) & GOTO statements which are helpful for the PDB
file to debug the code.
3.
Last but not the least; the RELEASE build code is not at all
OPTIMIZED in comparison to the DEBUG build code(as shown in
the link given above). Any optimization is done by JIT on
top of the IL code at the execution time.

if deleting the web.config u can't debug.means ur without 
web.config u don't debug the appliction.if ur build the 
application web.config not necessary.