Like Ruby on Rails, Merb is an MVC framework.  Unlike Rails, Merb is
ORM-agnostic, JavaScript library agnostic, and template language
agnostic, preferring plugins that add in support for a particular
feature rather than trying to produce a monolithic library with
everything in the core.  In fact, this is a guiding principle of the
project, which has led to third-party support for the ActiveRecord,
DataMapper, and Sequel ORMs.

In addition, it means that the core code in Merb is kept simple and
well organised.  This has multiple benefits.  It means it's faster
for one thing.  It's also easier to understand, maintain and extend.