Major difference between drupal 6 and drupal 7

Very important question is What is the Difference between Drupal 6 and Drupal 7.

Content Construction Kit (CCK)

Content Construction Kit (CCK) has moved to core in the Drupal 7. Custom data fields are available to nodes, users, comments and taxonomy terms.Fields are translatable.

Entity

Entities were introduced in Drupal 7. An entity would be one instance of a particular entity type such as a comment, taxonomy term or user profile or a bundle such as a blog post, article or product.

You can use entity_load to load any entity. Note, however, that the core does not provide a save or delete function, but thanks to Entity API module the missing pieces are added (entity_create(), entity_save(), entity_delete(), entity_view() and entity_access()).

Security

•     Modules can be updated via the web.
•     More secure log-in system.
•     More secure password system.
•     More secure implementation for scheduled tasks (cron.php).

Update Manager

Introduced an Update manger in Drupal 7 which perform updates directly through the administration interface.

File handling

• Files are now first class Drupal objects with file_load(), file_save(),and file_validate() functions and corresponding hooks.
• Files use PHP stream wrappers to enable support for both public and private files and to support pluggable storage mechanisms and access to remote resources (e.g. S3 storage or Flickr photos).
• Added a field specifically for uploading files, previously provided by the contributed module FileField.

Image handling

Improved image handling, including better support for add-on image libraries.
Added a field specifically for uploading images, previously provided by the contributed module ImageField.
Search

Imporoved search support language-aware searches

Database

• Added query builders for INSERT, UPDATE, DELETE, MERGE, and SELECT queries.
• Support for master/slave replication, transactions, multi-insert queries,delayed inserts, and other features.
• Added support for the SQLite database engine.
• Default to InnoDB engine, rather than MyISAM, on MySQL when available for greater scalability and data integrity.

jQuery

• Upgraded the core JavaScript library to jQuery version 1.4.2.
• Upgraded the jQuery Forms library to 2.36.
• Added jQuery UI 1.8, which allows improvements to Drupal's user experience.

Multisite

Better Support for Multisite Installations