Drupal is based on Hook system by which it interacts with modules. The modules interact with the core using hooks and add their own features or alter the existing features. Here are the commonly used hooks that are used in Drupal programming often.
Block related hooks
| hook_block_info | Define all blocks provided by the module. |
| hook_block_info_alter | Change block definition before saving to the database.Change block definition before saving to the database. |
| hook_block_configure | Define a configuration form for a block. |
| hook_block_save | Save the configuration options from hook_block_configure(). |
| hook_block_view | Return a rendered or renderable view of a block. |
| hook_block_view_alter | Perform alterations to the content of a block. |
| hook_block_view_MODULE_DELTA_alter | Perform alterations to a specific block. |
Cron Related Hooks
| hook_cron | Perform periodic actions. |
| hook_cron_queue_info | Declare queues holding items that need to be run periodically |
| hook_cron_queue_info_alter | Alter cron queue information before cron runs. |
Node related Hooks
| hook_delete | Respond to node deletion. |
| hook_form | Display a node editing form. |
| hook_insert | Respond to creation of a new node. |
| hook_load | Act on nodes being loaded from the database. |
| hook_node_info | Define module-provided node types. |
| hook_update | Respond to updates to a node. |
Installation related Hooks
| hook_install | Perform setup tasks when the module is installed. |
| hook_schema | Define the current version of the database schema. |
| hook_schema_alter | Perform alterations to existing database schemas. |
| hook_uninstall | Remove any information that the module sets. |
| hook_update_N | Perform a single update. |
Help related Hooks
| hook_help | Provide online user help. |
Permission related Hooks
| hook_permission | Define user permissions. |
Menu related Hooks
| hook_menu | Define menu items and page callbacks. |
| hook_menu_alter | Alter the data being saved to the {menu_router} table after hook_menu is invoked. |
Form related Hooks
| hook_form_alter | Perform alterations before a form is rendered. |
| hook_form_BASE_FORM_ID_alter | Provide a form-specific alteration for shared (‘base’) forms. |
| hook_form_FORM_ID_alter | Provide a form-specific alteration instead of the global hook_form_alter(). |
Theme related Hooks
| hook_theme | Register a module (or theme’s) theme implementations. |
| hook_theme_registry_alter | Alter the theme registry information returned from hook_theme(). |