Manifest is an XML based, database driven Content Management
System (CMS) for satisfying corporate communications needs,
including Internet and Intranet. 

(or)


Assembly manifest is a data structure which stores
information about an assembly .The information includes
version information, list of constituent files etc.

Assemblies data is stored in manifest
In manifest include
assemblies information
like version of assemblies
security information etc
manifest is stored in PE file that is portable Executable 
file in MSIL(Microsoft intermediate Language)
Manifest make assembly self describing.CLR use mainifest 
for execution

In .NET Framework Assembly metadata stored in Manifest.

Manifest contains all the metadata needed to do the
following things

   * Version of assemblies
   * Scope of assemblies.
   * Reference to resource and class
   * Assemblies manifest stored in Portable Executable (PE)
file or in Microsoft Intermediate Language (MSIL) code.