Confused about ‘respond_to’ vs ‘respond_to?’

Ruby treats ? and ! as actual characters in a method name. respond_to and respond_to? are different. ? indicates that this should respond with a true or false (by convention; this is not a requirement). Specifically:

respond_to? is a Ruby method for detecting whether the class has a particular method on it. For example,

@user.respond_to?('eat_food')

would return true if the User class has an eat_food method on it.

respond_to is a Rails method for responding to particular request types. For example:

def index
  @people = Person.find(:all)

  respond_to do |format|
    format.html
    format.xml { render :xml => @people.to_xml }
  end
end

However, in the RailsTutorial link you’ve provided, you’re seeing an RSpec method should interacting with RSpec’s respond_to method. This wouldn’t be available in your console, unless you run rails console test.