Starting from Drupal 7, you can now select nodes with conditions using the EntityFieldQuery class. The following are a few examples.

Select nodes of a specific content type written by a specific author and posted 1 hour ago.

$query = new EntityFieldQuery();
$query->entityCondition('entity_type', 'node')
  ->entityCondition('bundle', '<CONTENT TYPE>') // ex. article
  ->propertyCondition('uid', $uid) // node written by a specific user
  ->propertyCondition('status', 1) // published nodes
  ->propertyCondition('created', REQUEST_TIME - 3600, '>='); // created within the past 1 hour
$result = $query->execute();

/*
print_r($result)
================
Array
(
  [node] => Array
  (
    [529] => stdClass Object
    (
      [nid] => 529
      [vid] => 529
      [type] => <CONTENT TYPE>
    )

    [531] => stdClass Object
    (
      [nid] => 531
      [vid] => 531
      [type] => <CONTENT TYPE>
    )

  )
)
*/

Select nodes of a specific content type with a field_name = ‘eureka’.

$query = new EntityFieldQuery();
$query->entityCondition('entity_type', 'node')
  ->entityCondition('bundle', '<CONTENT TYPE>') // ex. article
  ->propertyCondition('status', 1) // published nodes
  ->fieldCondition('field_name', 'value', 'eureka', '='); // field_name = 'eureka'
$result = $query->execute();

Select nodes of a specific content type with user reference field associated to a user with uid = $uid

$query = new EntityFieldQuery();
$query->entityCondition('entity_type', 'node')
  ->entityCondition('bundle', '<CONTENT TYPE>') // ex. article
  ->propertyCondition('status', 1) // published nodes
  ->fieldCondition('field_user_ref', 'uid', $uid, '='); 
$result = $query->execute();

Usually if the conditions are the columns of the node table, use propertyCondition(). For other fields, use fieldCondition().

Done =)

Next: Drupal 7 – Get mulitple nodes using EntityFieldQuery and node_load_multiple()

Reference:

• EntityFieldQuery
• How to use EntityFieldQuery