Mage::getSingleton() methods, the latter checks if the requested class is already instantiated and if that is true – returns object from registry. Otherwise, class is instantiated and stored into registry.
To retrieve resource models you have similar set of methods come handy, they are
Mage::getResourceSingleton(). And if you have a model object, its accompanying resource module can be retrieved using
Let’s get to the third whale – the collections. For the sake of simplicity collections are set of models, if a model presents table row as object, a collection presents a set of rows. Getting a collection is as easy as
Mage::getModel()->getCollection(). There is, however, a bit shorter notation
Mage::getResourceModel('module/model_collection')which is basically the same, if you take a look at
Mage_Core_Model_Abstract::getResourceCollection()method, but allows you to skip instantiating the model class.
I hope this brief overview of models and related stuff was useful for you and if there are any questions or unclear points feel free to let us know in the comments.