Merb was a lightweight, modular framework made up of a small merb-core and then merb-more, a collection of 20ish gems that provided various bits of functionality, most of which were problems that Rails solved in its core. Stuff like merb-helpers. If you wanted fancy helper methods, you required the merb-helpers gem. If you didn't care, you would not require that and have a slightly more efficient app. Similarly, it didn't have an ORM baked in--you'd use some gem like DataMapper (with an additional merb_datamapper gem that told Merb how to integrate with DataMapper).

Padrino seems to have the same setup of a minimal core with lots of modular plugins, but its equivalent to merb-core is sinatra. Which is awesome because sinatra is great and this way they don't have to reinvent the wheel.