Idol worship - Believing in many gods.

It is the most apparent kind of Shirk. All religions in which people worship many gods or idols or anyone other than God or associate God with anyone, are polytheistic religions. The followers of these religions are Mushriks.
There are many religions which practice idol worship.

Zoroastrians believe in two gods - god of evil (darkness) and god of goodness (light).

Buddhists do not believe in God. But Buddhists worship the statue of Gautam Buddha as their god. So do the followers of Jainism. They both are polytheistic religions.

In Hinduism, there are three main gods, and traditionally 33 million other gods and goddesses. In order to call themselves monotheistic, they say that behind all these gods, there is also a single spiritual entity called Brahma.

Christians also believe that God is present in three forms. This is known as trinity i.e. they believe that in one God there are three Divine persons - the Father, the Son, and the holy Spirit or holy Ghost. This means that Christians divide god into three par ts, Father - Son -Holy Spirit, and unite them together to make one single god. They use paintings, statues, and other art to depict their sacred figures and stories from the Bible.
In Eastern Europe and the Near East, worshipers give special reverence to icons (images of Jesus Christ or the saints).
Christians known as iconoclasts opposed the use of images because they considered it idolatry. But other Christians argued that such images (icons) are merely symbols to help worshippers think of God.
A dispute called the iconoclastic controversy raged between the two sides during the AD 700’s and early 800’s. The iconoclasts destroyed the pictures and statues in many churches.
A similar disagreement over the use of images broke out between Protestants and Roman Catholics during the 1500’s in which Protestant destroyed many statues and images which were being worshipped by the Catholics.

The ancient Greeks and Romans had polytheistic religions. They assumed sky, stars, sun, planets, moon, and many other things as gods and used to worship them. They named each day of the week and month by their god such as Sunday (sun), Monday (moon).

The ancient Egyptian people considered their kings ( Pharaohs ) to be living gods.

In the Shinto religion of Japan, gods are thought to reside in particular trees, rocks and streams.

Before World War 11 ( 1935-1945 ), the Japanese people honored their emperor as god. On September 2, 1945 Emperor Hirohito who was considered and worshipped by Japanese as their god, officially surrendered and announced his defeat. Thousand of Japanese were weeping seeing their god in such a humiliated condition. On Jan. 1, 1946, Emperor Hirohito renounced all claims to the divinity (god) that formerly had been claimed for him. The constitution of 1947, which he approved, changed him from sovereign to “symbol of the state”

The Gallis or Ghullat (extremists), among Muslims, form another group of Mushriks. They are also called Mufawwiza. They believe that Allah has delegated His powers or entrusted all affairs to the Holy Imams. According to them, the Imams give life and death, and they also give us sustenance. According to all Shia Ulama, any person who has this belief or similar views about Imams is a Mushrik.

Islam considers all such religions as polytheistic and believes in the absolute oneness of God.

Quran has rigorously condemned all these religions and tells that the followers of these religions will be severely punished.

Thus, all religions, except Islam, are polytheistic religions. Islam is the only religion which strongly believes in pure monotheism (i.e., absolute oneness of God).