An exception is an instance of the class Exception or a descendant. In ruby to rescue exceptions we do something like this:
 

def process_request!
  #some code here
  rescue
    Rails.logger.error { "#{e.message} #{e.backtrace.join("\n")}" }
    puts "Exception caught"
end

OR 

rescue => e

In both ways we rescue standard errors of exception class and is a good way to handle errors. 

However sometimes we make terrible mistake by rescuing Exception like this:
rescue Exception => e
It swallows all type of errors. Exception is the root of the exception class hierarchy in Ruby.
 
 rescue => e is shorthand for rescue StandardError => e

Handling exception globally:

class ApplicationController < ActionController::Base
 
  rescue_from ActiveRecord::RecordNotFound, :with => :handle_exception
 
  def handle_exception
    flash[:error] = error.message
    redirect_to root_path
  end
end