The belief in Prophets and Messengers I s an important doctrine of Islam. There should be respect for ealier prophets and Messengers like David, Moses and Jesus. Whenever the muslim pronounces the name of any of them e.g. Isa, he is duty-bund to pay his respect to him by saying "Alayhi as-salam" (May peace be upon him).
God sent Messengers or Prophets to preach the Unity of God at all times and to all people. Most of them were persecuted by the majority of their people.
However, there is a difference between a Messenger (Rasūl Allah) and a Prophet (al-Nabi). A messenger is the Prophet who is sent to give a divine message to reform mankind. He is given a devine book. All Prophets do not get the book, and therefore, all prophets are not Messengers.
Prophethood is a bounty and a favour from God given to him whom Allãh pleases. The chief objective of sending the prophets is to reform the spoilt society. The prophet brings the message from Allãh and not from any Other person.
The exact number of prophets is unknown, but the number 124,000 has been suggested. The last of all the prophets is Muhammad who is refferred to as the seal of the prophethood (khãtim al-Mursalin).