Muslims believe that Muhammad is the last and greatest of all Prophets because he was a great example of how humans should live and all must strive to have the same experience that Muhammad had.
Muhammad first met Allah when he was 40 years old and was meditating in a cave on a nearby mountain when Allah appeared in the form of a prophet of his, Gabriel. Gabriel came and ordered him to read from a scroll. Although Muhammad was illiterate, he was still able to recite the words from the scroll. It was from then on that Muhammad had complete faith in Allah.
The city of Mecca is sacred to Muslims because it was in Mecca that Muhammad first preached Islam. Jerusalem is sacred to Muslims because this was where, according to Muslim that Muhammad ascended into heaven upon a white horse.
The Qu"ran is significant to Muslims because it is the sacred books about the teaching of the prophet Muhammad. The Sunna is significant because it is a collection of six collections of traditions, known as the six Sound Books, which contain a few thousand of the hundreds of thousands of traditions that had been in circulation. The Ijma is significant the majority of Muslims because it is the ruling describing what is permitted and what is forbidden after the passing of the Prophet.
Within the mosques at Mecca there are no statues or paintings because during the lifetime of Muhammad, he felt he should cleanse the Ka"aba and dedicate it solely to Allah. From that moment on it became the central point of sacred space for every Muslim.
There are two main festivals that Muslims participate in. The first one would be Festival of Eid ul-Fitr, which is held at the end of Ramadan, which is the thirty day fast there are huge celebrations. This is a very special occasion and it would seem out of place for any one to go around not dressed in their very best clothes. Everyone goes to the mosque and sit in prayer together.