In answer to this question, Maulana replied:-
For Amr Bil Maroof (Enjoining Good) there is a need for Salahiyyat (Competence) and for Nahy Anil Munkar there is a need within the Ummah for Istidaad (Willingness and inclination). Until and unless the attributes of Salahiyyat and Istidaad are not developed, a great multitude of this Ummah will mock and belittle the divine laws of Shariah the moment the mission of Amr Bil Maruf Wa'Nahy Anil Munkar is implemented and the Fatawas (religious verdicts) of permissible and impermissible are passed. And naturally, if a person commits a sin whilst considering it to be a sin, it won't be as great an offence as it would be in the case of mocking and belittling the most seemingly trivial law of Shariah. The gravity of such an offence can be gauged from the fact that ridiculing the most seemingly insignificant law of shariah render's one as a disbeliever. Hence, if Amr Bil Maroof Wa'Nahy Anil Munkar are propagated without the Salahiyyat and Istidaad of the Ummah, there is a fear of the Ummah landing itself in Kufr as a consequence to the mocking and belittling of the laws of Shariah.
Similarly, Tabligh is the name awarded to the propagation of Islam. Today our own condition is such that we lack the attributes of Salaah and Falaah ( piety and spiritual prosperity) without which Tabligh is not possible. When non-muslims witness the deeds and words of this Ummah, they find them to be in total conflict with the teachings of Shariah. How can our Shariah have an impact on others when the Ummah itself is devoid of the laws of Islam. Hence prior to Tabligh amongst non-muslims, the Ummah is compelled to develop the attributes of Salaah and Falaah to a degree of excellence.
The current efforts amongst the Ummah itself with the label of Tabligh are all being implemented to promote activities of Dawah. And this activity in turn will lead to the Salahiyyat of Amr Bil Maroof and the Istidaad of Nahy Anil Munkar. Owing to this Dawah mission (Tablighi Jamaat) an awareness and a spirit of Deen (faith) will be created in the Ummah and this is the activity which in turn will bring about piety and spiritual prosperity. As a consequence to piety and spiritual prosperity, Tabligh will advance steadily whilst complying with the principles and rules of conduct.
So, in brief, the medium of Dawah (Tabligh) is employed today to engender the attributes of salahiyyat and Istidaad and to develop an awarenes and quest together with piety and spiritual prosperity without which the activities of Amr Bil Maroof and Nahy Anil Munkar cannot be instituted. Hence our current endeavour is a Dawah. "
The endeavour of Tabligh is an endeavour of Dawah and Ijaabah (invitation and compliance).
Taken with Jazakallah from http://engagersindawah.blogspot.in/2008/02/difference-between-amr-bil-maroof.html