Merb and Ruby on Rails are MVC web application frameworks written in and for Ruby. Both have similar directory structures and concepts. Ruby on Rails (RoR) is more featured while Merb aims to be simpler and more modular. Merb and RoR are merging development efforts and will be one product for Merbv2/RoRv3

Merb

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.

Merb is also thread-safe and was originally engineered to handle multiple file uploads concurrently.

Rails