What is the difference between $form & $form_state in Drupal

I am working with drupal 7 really its crazy but some how i confused  in drupal $form & $form_state

I discussed with many people now i got some clarification on this $form & $form_state values.$form defines the form, $form_state carries information about the processed form.

How to validate froms in Drupal 7?

Drupal Form validations

Here you can get the form values from form_state 

Below is the simple example for form validations  

function test_form_validate($form, &$form_state) {
  if ($form_state['values']['name'] == '') {
    form_set_error('name', t('You must select a name for this group of settings.'));
  }
}

Drupal 7 Basic form with submit handler

Bellow code tells about the basic form submit in drupal7

<?php
function my_module_menu() {
  $items = array();
  $items['my_module/form'] = array(
    'title' => t('My form'),
    'page callback' => 'my_module_form',
    'access arguments' => array('access content'),
    'description' => t('My form'),
    'type' => MENU_CALLBACK,
  );
  return $items;
}

function my_module_form() {
  return drupal_get_form('my_module_my_form');
}

/*
  # Sample registration form 
*/
function my_module_my_form($form_state) {
  $form['name'] = array(
    '#type' => 'fieldset',
    '#title' => t('Name'),
    '#collapsible' => TRUE,
    '#collapsed' => FALSE,
  );
  $form['name']['first'] = array(
    '#type' => 'textfield',
    '#title' => t('First name'),
    '#required' => TRUE,
    '#default_value' => "First name",
    '#description' => "Please enter your first name.",
    '#size' => 20,
    '#maxlength' => 20,
  );
  $form['name']['last'] = array(
    '#type' => 'textfield',
    '#title' => t('Last name'),
    '#required' => TRUE,
  );
  $form['year_of_birth'] = array(
    '#type' => 'textfield',
    '#title' => "Year of birth",
    '#description' => 'Format is "YYYY"',
  ); 
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => 'Submit',
  );
  return $form;
}

function my_module_my_form_validate($form, &$form_state) {
    $year_of_birth = $form_state['values']['year_of_birth'];
    if ($year_of_birth && ($year_of_birth < 1900 || $year_of_birth > 2000)) {
      
  form_set_error('year_of_birth', 'Enter a year between 1900 and 2000.');
    }
}

 function my_module_my_form_submit($form, &$form_state) {
    drupal_set_message(t('The form has been submitted.'));
}

?>

Drupal 7 Confirm Password form alter example

Description: Format a pair of password fields, which do not validate unless the two entered passwords match.

 

<?php
$form['pass'] = array(
  '#type' => 'password_confirm',
  '#title' => t('Password'),
  '#size' => 25,
);
?>

How to create checkboxes in Drupal form alter?

Description: Format a set of checkboxes. #options is an associative array, where the key is the #return_value of the checkbox and the value is displayed. The #options array cannot have a 0 key, as it would not be possible to discern checked and unchecked states.

 

<?php
$form['high_school']['tests_taken'] = array(
  '#type' => 'checkboxes',
  '#options' => drupal_map_assoc(array(t('SAT'), t('ACT'))),
  '#title' => t('What standardized tests did you take?'),
...
),?>