In the preceding section, you wrote a tiny script and then ran it by passing the name of the file to the Ruby interpreter. If you think about it, you already cut the typical development cycle in half by skipping the compilation portion. That's good, for sure, but you still have a couple of issues to deal with.
Imagine that you are writing a large program and would now like to incorporate a new functionality. Wouldn't it be nice to be able to try it out before you actually start to change your program? You could create a new file, write the snippet of code to test, and then run it as usual through your editor. That works, but Ruby is all about having fun and that approach sounds a bit tedious and not so immediate. A better way to go is to use a tool called irb (Interactive Ruby).
Our second point to consider is that you are brand new to the world of Ruby. You don't know what methods are available for a given class or how to use them. You could use Google, and the API for Ruby's Core and Standard libraries is available online in several places (for example, http://www.ruby-doc.org). But it's not as immediate. To help you out, there's ri (Ruby Interactive).
The two tools have very similar names, which can be quite confusing. Most rubyists prefer to simply call them irb and ri.
irb and ri should become "your best friends" if you wish to learn Ruby. But don't think that they're only for beginners. They are used by Rubyists of all levels and even by Matz ...