To be able to carry out the actual meditation-practice of the deity of the initiation we must receive the reading transmission (lung) of the actual practice from our teacher.
This takes the form of the teacher reciting the words of the Buddhist teaching or text to us, conveying the blessing of the lineage through which the teaching has been transmitted.
Although in vajrayana you must receive reading transmission for any text you wishes to study or practice, in the sutra teachings this is not considered to be quite as necessary.
However, the transmission for many sutra texts still exists and it is certainly beneficial to receive such transmissions since it nurtures our realization.
The third category of vajrayana teaching, following initiation and reading transmission, is instruction (khrid) in the actual techniques contained in the meditation practice.
In many, though not all, cases such instructions are now in the form of commentaries based on the discourses of great masters of the lineage with further oral clarification from our lamas.