Active Records implement validation by overwriting Base#validate (or the variations, validate_on_create and validate_on_update). Each of these methods can inspect the state of the object, which usually means ensuring that a number of attributes have a certain value (such as not empty, within a given range, matching a certain regular expression).

Example:

class Person < ActiveRecord::Base
  protected
    def validate
      errors.add_on_empty %w( first_name last_name )
      errors.add("phone_number", "has invalid format") unless phone_number =~ /[0-9]*/
    end

    def validate_on_create # is only run the first time a new object is saved
      unless valid_discount?(membership_discount)
        errors.add("membership_discount", "has expired")
      end
    end

    def validate_on_update
      errors.add_to_base("No changes have occurred") if unchanged_attributes?
    end
end

person = Person.new("first_name" => "David", "phone_number" => "what?")
person.save                         # => false (and doesn't do the save)
person.errors.empty?                # => false
person.errors.count                 # => 2
person.errors.on "last_name"        # => "can't be empty"
person.errors.on "phone_number"     # => "has invalid format"
person.errors.each_full { |msg| puts msg }
                                    # => "Last name can't be empty\n" +
                                         "Phone number has invalid format"

person.attributes = { "last_name" => "Heinemeier", "phone_number" => "555-555" }
person.save # => true (and person is now saved in the database)

An Errors object is automatically created for every Active Record.

Please do have a look at ActiveRecord::Validations::ClassMethods for a higher level of validations.