From prodefence.org
Obfuscar is a basic obfuscator for .NET assemblies. It uses massive overloading to rename metadata in .NET assemblies (including the names of methods, properties, events, fields, types, and namespaces) to a minimal set, distinguishable in most cases only by signature.
For example, if a class contains only methods that accept different parameters, they can all be renamed ‘A’. If another method is added to the class that accepts the same parameters as an existing method, it could be named ‘a’.
Basically, Obfuscar scrambles the metadata in a set of assemblies. It renames everything to the minimal set of names that can be used to identify them, given signatures and type information. Since these new names are shorter than the old ones, it also dramatically shrinks the executable size.