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Question: How does Validation works in Ruby on Rails?

Validation means checking to see if data  is good before it is stored in the database.

During signups and other such user input cases you want to check and be sure that the data is validated. In the past developers would often put this type of validation logic as triggers in the database.  

In an MVC architecture one can do validations at each level. 

You can do validations in the controllers but it is usually a good idea to keep your controllers skinny.

Views suffer from the javascript limitation because javascript can be disabled on the client side so they are not completely reliable.

The best way to manage validation is to put it in the model code. This model code is really the closest as you can be to the database and works very well for Rails applications.

Here are a few validation examples:

class Person < ActiveRecord::Base
  validates :name, :length => { :minimum => 2 }
  validates :points, :numericality => { :only_integer => true }  # only integer
  validates :age,  :numericality => { :greater_than => 18 } # greater than 18
  validates :email, :uniqueness => true
  validates :email, :confirmation => true  # this is to validate that the two email fields are identical
  validates :email_confirmation, :presence => true # this is to validate that the email confirmation field is not nil

In your view template you may use something like this:

 <%= text_field :person, :email %> <%= text_field :person, :email_confirmation %>