I am `fairly new to ruby and ROR`. I have a couple of questions on migration. I had a look on the guides, but its confusing for me. 

<http://guides.rubyonrails.org/migrations.html>

```
class CreateUserModel < ActiveRecord::Migration
  def self_up 
    create_table :users do |t|
      t.column :username, :string
      t.column :email, :string
      t.column :password_hash, :string
      t.column :password_salt, :string
    end

  def self_down
    drop_table :users
  end
end

```

1. When i searched google, i found on this command to run migration `rake db:migrate
VERSION=20098252345`. I am confused on what is this VERSION doing
with the command. I just have created a migration 0001\_create\_user\_model.rb
2. The above is my `migration` table. What does it create when i
run/execute this file. Does it create tables and models for me?