If you look back to the previous code example, you might notice that we've not only instantiated a Product model, but also we've called the getCollection() method. The getCollection() method is part of the Mage_Core_Model_Abstract class, meaning that every single model inside Magento can call this method.
A Magento collection is basically a model that contains other models. So, instead of using an array for holding a collection of products, we could use a Product collection instead. Collections not only provide a convenient data structure for grouping models but also provide special methods that we can use to manipulate and work with collection of entities.