They believe in the one true God (Allah), and his total rule over humanity, a day of judgement to come, and life after death. There is no sense of assurance about receiving mercy from Allah on the final day, and so a life lived in merit, in accordance with the five pillars Islam is necessary.
Muslims believe that Jesus was a prophet, although God's ultimate revelation of himself was to Mohammad, through the Angel Gabriel, recorded word for word in Arabic in the Quran.
Here are the five pillars of Islam that Muslims must uphold:
1/ The Creed
'There is no God but God (Allah), and Mohammad is his messenger.' Reciting this creed sincerely in the presence of a witness is what makes you a Muslim.
During daylight hours in the month of Ramadan, Muslims may not eat, drink, smoke or have sex.
3/ Giving to the Poor
True Muslims are expected to give 2.5% of their income to the poor.
Muslims pray in the direction of Mecca at five set times of the day. These are set prayers in Arabic, with more optional prayers available.
All Mulsims who are physically able to, are expected to make a pilgrimage to Mecca at least once during their life time. Those who give money to enable others to travel believe that it gains them merit with God.
This is the briefest of all summaries of belief possible. It is not an endorsement or an attempt to highlight flaws - but even a basic understanding of sincerely held faith can open doors of understanding, perhaps breaking down some of the misconceptions which lead to fear, ignorance and sometimes unhelpful behaviour from Christians towards Muslims.