What do people believe in?

You name it, and you'll find somebody who believes it.

Some people today worship Zeus. (For real)

Others believe in religions, for which there is little or no evidence.

Some people believe in science and reasoning (including many some religious people).

Delusional people believe practically anything, including their own divinity.

Most (but not all) people believe that the world as we experience it is real, and so are other people. That seems like a good place to start, because without that assumption, you may as well just give up on believing anything. Well altogether people usually believe in a religion or something that tells the person the purpose and creator of life and how to live life. That religion etc guilds the person throughout their whole life, it's like a guideline to live a better life. So generally people believe in whatever they want to believe in, whether it is a religion or not.
Well, that depends on who you are talking about. Christians believe in Jesus Christ, God the Father, Jesus the Son, and the Holy Spirit. (God is a trinity, 3 heads in one)

Among many other believe systems, Jews believe in the teachings of the Prophets, and the explanations of Rabbis and biblical scholars as recorded in the Jewish Holy texts. Muslims believe in the teachings of Mohammad as recorded in the Islamic Holy texts and traditional teachings, Buddhists believe in the teachings of The Buddha and related philosophies, and Hindus believe in the teachings of the Hindu Holly texts and traditional teachings.

Some are said to believe in Satan, the devil, but generally consider that to be a negative view of their beliefs as seen by others who do not share them.