The flash provides a way to pass temporary objects between actions. Anything you place in the flash will be exposed to the very next action and then cleared out. This is a great way of doing notices and alerts, such as a create action that sets flash[:notice] = "Successfully created" before redirecting to a display action that can then expose the flash to its template. Actually, that exposure is automatically done. Example: 

  class WeblogController < ActionController::Base
    def create
      # save post
      flash[:notice] = "Successfully created post"
      redirect_to :action => "display", :params => { :id => post.id }
    end

    def display
      # doesn't need to assign the flash notice to the template, that's done automatically
    end
  end

  display.erb
    <% if flash[:notice] %><div class="notice"><%= flash[:notice] %></div><% end %>

This example just places a string in the flash, but you can put any object in there. And of course, you can put as many as you like at a time too. Just remember: They’ll be gone by the time the next action has been performed.

See docs on the FlashHash class for more details about the flash.

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November 16, 2008 - (>= v2.1.0)
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Application Helper for Fading Flash Messages

A simple helper method for showing the flash message. Includes optional fade in seconds (view needs javascript_include_tag defaults if you desire fade effect): 

  def show_flash_message(options={})
    html = content_tag(:div, flash.collect{ |key,msg| content_tag(:div, msg, :class => key) }, :id => 'flash-message')
    if options.key?(:fade)
      html << content_tag(:script, "setTimeout(\"new Effect.Fade('flash-message');\",#{options[:fade]*1000})", :type => 'text/javascript')
    end
    html
  end

simply call in your views then using: 

  <%= show_flash_message(:fade => 4) %>