The fundamental belief of Islam is that there is no god but Allah, and Mohammed is the prophet of Allah. This is called the 'Shahadah', or 'Profession of Faith', and expresses the absolute monotheism which defines Islam.
Other key points of belief are expressed in the Aqida, the Six Articles of Faith. Although there are some variations within Islam, they are generally expressed as:
1) Belief in God (Allah), who alone is worthy of worship (tawhid)
2) Belief in the Prophets (nabi) and messengers (rusul)
3) Belief in the angels (mala'ika)
4) Belief in the books of God (e.g, the Quran)
5) Belief in Judgement Day (qiyama) and in the resurrection
6) Belief in fate (qadar)* --> (this one tends to be subject to a little variation in interpretation)
Muslims must also uphold the Five Pillars of Islam, which are:
1) Reciting the Shahadah
2) Praying five times a day in the appropriate manner
3) Giving zakat (alms) to the poor and needy
4) Fasting during Ramadan
5) Making the Haj (pilgrimage) to Mecca at least once if one can afford it
There is also the important concept of haram (disallowed) and halal (allowed). That which is regarded as impure is haram, that which is regarded as pure is halal. These include many practices ranging from diet to interactions between the sexes. For instance, alchohol and pork are haram.
In addition to strong beliefs in justice, mercy, the family, and peace, that's basically Islam in a nutshell. (= If you want a better idea, you should ask an educated Muslim, he or she will have the best idea.
My apologies if what I've said is innacurate, please don't hesitate to correct me.
