DLL hell, basically the versioning problem of the assembly. 
Which is solved by .net automatically.

DLL Hell refers to the set of problems caused when multiple 
application attempt t share a common component like DLL or 
COM

DLL HELL was there in earlier version of .Net .With the 
latest Version .Net handles this automatically.
It arised due to two main reasons.:-
1.> While the dll are named the name may match with those 
existing by default in windows folder or if the dll has not 
been updated after adding an additional functionality then 
the application will throw error and this resulted in DLL 
hell.This can be avoided by either updating the DLL or by 
changing the version in web.config through strong tag.
2.> There may be a side effect with dll as particular 
memory reference may get deleted then the application will 
throw error and result in DLL hellas it wont get that 
referenced memory required in the application

if one application required new version of dll it would 
update the dll during application installation as result of 
this update other program using the dll may no longer 
function properly. This type of proplem is known as DLL HELL