What is mage in magento ,,explain following and also list all these kind of function or model used in magento

Question

And also explain following function:

Mage::getModel('catalog/product');
load();
$category = Mage::getModel('catalog/category');
$category->load($categoryid);
$collection = $category->getProductCollection();
$collection->addAttributeToSelect('*');

Answer 1

The Mage is a super-class used to obtain access to the main methods/processes used in Magento.

Mage::getModel('catalog/product'); returns new (empty) instance of the Mage_Catalog_Model_Product class.

load() - the regular method of AbstractModel in Magento which loads corresponding model's data from the database.

$category = Mage::getModel('catalog/category'); - get new category model and store it in the $category variable. Later you can add a data to it and save it using save().

$category->load($categoryid); - loads from the databse specific category in case when the $category is instance of AbstractModel.

$collection = $category->getProductCollection(); - returns the collection of a products for this category. Each of them is instance of Mage_Catalog_Model_Product.

$collection->addAttributeToSelect('*'); - select all of the available attributes during future load process. In case when the collection was already loaded will give no effect.