Profiling a ruby / ruby on rails application

I am looking to understand conceptually what all goes in profiling a ruby or ruby on rails program (e.g. memory usage, speed of request dispatching, speed of connecting with external programs like DB) and what are the best tools (at a conceptual and fundamental level) that are available.

What is Ruby prof?

ruby-prof is a profiler for MRI Ruby. Its features include: Speed - it is a C extension and therefore many times faster than the standard Ruby profiler. Measurement Modes - ruby-prof can measure program wall time, process time, object allocations and memory usage.

I don't know if it helps, but I read somewhere article saying, that Java is the best profiler for Ruby on Rails applications. The idea was to run them on JRuby and use JProfiler or other Java profiling tools to profile your Rails application. Another possibility that I know of might be the Relic RPM, commercial application especially for profiling Rails applications.