The base classes are fully documented by let's make a summary of the API.
The basic methods are dbLoad and dbLoadAll.
This methods support only a limited set of conditions and queries are the prefered way of loading data from the database.
dbLoad loads one object or returns null if it doesn't exist.
dbLoadAll returns a query, queryresult or collection of objects that comply with the condition. By default it returns a query.
Conditions is an array of pairs attribute => [operator => value].
More information about the format can be found here.
An array where keys are attributes and values are "ASC" or "DESC".
Limit can be a number (size of returned elemetns) and an array of the form [offset, size].
You should only use dbSave. This methos checks if the model object already exists in the database and, if that's the case, updates it instead of inserting a new tuple.