What's the difference between ElementCriteriaModel and DbCommand?
DbCommand represents a SQL statement to execute against a database.

ElementCriteriaModel is a model just like any other model in craft/app/models, but it also happens to make it easy to search for Elements in the database.

By setting all of the criteria for the elements you're searching for and calling the find() method, that will invoke craft()-&gt;elements-&gt;findElements($this);, which in turn will create a DbCommand object custom built for the ElementCriterialModel parameters that you set.

So think of ElementCriteriaModel as an abstraction on top of DbCommand that makes it easy to search for Elements.