What Islam teaches
Islam is built on one central claim: that there is one God, who created everything, who is not part of creation, and who has communicated with humanity through prophets. Muslims believe that communication reached its final form in the Quran, given to Muhammad in Arabia in the seventh century. Sunni Muslims usually summarise belief as six items: God; angels; the books God revealed; the prophets who brought them; a day on which people are raised and judged; and God's decree, meaning that nothing happens outside God's knowledge and will. Practice is usually summarised as five: the declaration of faith, the five daily prayers, an annual obligatory payment to the poor, fasting the month of Ramadan, and the pilgrimage to Mecca once in a lifetime if a person can afford it and is able.
What you may observe
In practice most of this is invisible. What a colleague or neighbour actually does is step out for a few minutes several times a day, decline a drink, ask what is in the food, fast for a month once a year, and possibly travel to Mecca once in their life. Belief in angels or in a day of judgement does not produce anything you can see, and most Muslims will not raise it unless you ask.
Why it matters
The six items are not a creed recited at a service; they are the frame that makes the practice make sense. Prayer five times a day is tedious as a habit and coherent as an appointment with someone. Giving a fixed share of savings away every year is a strange tax and a reasonable duty if wealth is held in trust rather than owned outright. If you want to understand why a Muslim does something, the belief underneath it is usually the shortest route.
What varies
The six-item summary is a Sunni formulation. Twelver Shia Muslims — the majority in Iran, Iraq and parts of Lebanon and the Gulf — set out the foundations of religion differently: God's oneness, God's justice, prophethood, the imamate (the authority of a line of leaders descended from the Prophet), and the resurrection. The disagreement about the imamate is real and old; it is not a matter of emphasis. Ismaili and Zaydi Shia, and the Ibadi Muslims of Oman, differ again. Sufism is a further thing people confuse with a sect: it is a tradition of discipline and inward practice found inside Sunni and Shia Islam, not a third branch.
A common misconception
That Islam is a set of rules with a theology bolted on afterwards. It is usually presented that way, including by Muslims in a hurry, because the rules are the visible part. The rules are downstream. A second misconception is that the five pillars are all there is to practice — they are a summary of obligations, not an account of a religious life, and honesty, keeping promises and how you treat your parents occupy far more of Islamic ethical writing than any pillar does.
Compared with your context
If you know Judaism or Christianity, the shape will be familiar: one God, prophets, scripture, a final accounting. The sharpest difference is what happens at the centre. Christianity places a person — Jesus, understood as God incarnate — at the centre of how God is known. Islam places a text, and holds that Jesus was a prophet and not divine. Judaism and Islam are closer on that point and differ on whether revelation continued after the Hebrew prophets.
Ask more
Anything about what you personally should do — a decision about family, money, health or civil status — is not what an article like this is for. Those go to a qualified person who can ask you questions back. The reviewed answers on this platform will route you to one rather than guessing.