Judaism is based on strict monotheism*. This doctrine expresses the belief in one indivisible God.

The prayer par excellence is the ShemaYisrael:

Hear O Israel,

the Lord is our God,

the Lord is One.

(Deuteronomy 6:4).

So the Jews conceive God as eternal, the creator of the universe, and the source of morality. God has the power to intervene in the world. So God is an actual ontological reality, and is not merely a projection of the human psyche. Maimonides, for instance, describes God as:

A Being, perfect in every possible way, who is the ultimate cause of all existence.

All existence depends on God and is derived from God.

(Wikipedia: God_in_Judaism)

God is the creator of the universe: 

According to the Biblical account, the world was created by God in six days. While most of the Modern Orthodox, Conservative, and Reform authorities feel that the six days should be interpreted as “stages” in the creation of the universe. In this way Judaism would feel out of place with the current scientific model. Orthodox Jews believe in the omnipotent, omniscient God.

God is One: 

The idea of God as a duality or trinity is heretical – “it is considered a kin to polytheism” as Wikipedia holds. “[God], the Cause of all, is one. This does not mean one as in one of a pair, nor one like a species (which encompasses many individuals), nor one as in an object that is made up of many elements, nor as a single simple object that is infinitely divisible. Rather, God is a unity unlike any other possible unity.” This is referred to in the Torah.

God is personal: 

Most of classical Judaism views God as personal. According to them, humans have a relationship with God; God has a relationship with us. Much of the midrash, and many Jewish prayers imagine God as caring about humanity just as we care about God.

Monotheism* expresses the belief in one indivisible God. In contrast, Polytheism concept has multiple gods or God taking multiple forms (for example the Trinity in Christianity).