Lookup override is nothing but just simply overriding the default sql which is generated by lookup by the runtime, default SQL contains SELECT and ORDER BY caluse and the order by clause ord...
Source definition in database is simply the table definition, but the source qulifier is one of the transformation in informaitca from which we can specify the data selection criteria to the database....
A source definition database contain the datatypes that are used in the orginal database from which the source is extracted. where the source qualifier is used to convert the source definition datatypes to the informatica datatypes. which is easy to work with.