Difference between session and parameters in controller class

I am looking at an example of rails for shopping carts and in the ApplicationController class I see code like this:

class ApplicationController < ActionController::Base
  protect_from_forgery

  private

    def current_cart 
      Cart.find(session[:cart_id])
    rescue ActiveRecord::RecordNotFound
      cart = Cart.create
      session[:cart_id] = cart.id
      cart
    end
end

      

        
        
        
      

    

so it uses Cart.find (session [: cart_id])

Then I go to carts_controller.rb and CartController category and I see code like this:

 def update
    @cart = Cart.find(params[:id])

    respond_to do |format|

      

        
        
        
      

    

so Cart.find (params [: id]) is used here

But I can't figure out why we were using session to pass parameters to AppController, but we were using normal parameters in CartController and could we have used them using swithc? or is this how rails work and always the session goes to the AppController? It would be greta if someone can explain this in more detail