Islam is the religion of all Prophets from Adam to Muhammad (peace and blessings of Allah be upon all of them). All of them were Allah’s Messengers and Prophets who were sent to call humankind to believe in one God and guide people to the Way of God, the right way.
They conveyed to people what was revealed to them by God (Allah) through the Angel Gabriel. Gabriel revealed to them the teachings and instructions by which they can guide people and illuminate their path, and save them from darkness of ignorance to the light of true faith.
All the Prophets (peace and blessings be upon them all) were concerned in guiding people to worship Allah alone, without ascribing others along with Him. That is to say, the message of Muhammad (peace and blessings be upon him) completes all previous Divine messages.
With regard to the other question, there is neither a religious nor a political leader for Muslims nowadays due to many historical factors. Until 1924 AD Muslims had a general leader called the Caliph, i.e. a reverend leader from both religious and secular perspectives.
In addition, there are Imams (learned religious scholars) who call for Islam and teach people its fundamentals and rulings, in the similar way the Pope does to Catholics. Nevertheless, Imams can neither legalize what Allah forbids nor prohibit what He, Glorified be He, has made lawful. Their role is just to clarify and teach what has been already established.
Imams can sometimes use their own discretion to drive the rulings and teachings included in the Islamic texts, but they receive neither Divine Revelation (wahy) nor secular instructions.
Islam is the religion of simplicity and tolerance. The most manifest proof of this is that Christians and Jews have lived in security and peace in Muslim countries. Throughout the history of Islam, Muslims did not harm the People of the Book, nor destroyed their churches and monasteries. This is in contrast to what the Spanish Inquisition did to the Muslims and their civilization that lasted eight centuries in Andalusia.