Judaism is a monotheistic faith, meaning that Jews believe there is only One God. Often this God is beyond our ability to comprehend, but God is nevertheless present in our everyday lives. How individual Jews choose to understand this manifestation of the divine varies. Some connect with God through prayer, others see the divine in the majesty of the natural world, others may not think about God on a daily basis. Each individual's relationship with God is unique and personal.
Muslims monotheism is closer to that of Judaism than Christianity, rejecting the Christian concept of a "Trinity." Muslims believe that God, creator of all of existence, is just, omnipotent, and merciful. Muslims also reject the anthropomorphization of God in other monotheistic religions like Christianity or Judaism. For Muslims, God is completely "other" — God does not talk, does not walk and does not do anything like humans.
Christianity (quoting myself)
Christianity isn't as easy to define - in fact there are many religious groups who consider themselves Christian but have very different beliefs. Some of those are Catholic, Protestant, Christian Scientist, and Rosicrucians. Very generally speaking, Christians believe in a single God and is a Monotheistic religion. They generally believe in the trinity which is the God, Jesus, and the Holy Spirit are homoousios (the same substance).
All three believe that their God is the God of Abraham. Jewish people believe that Christians believe in the same God that they do and Christians also believe that they believe in the same God as Judaism. Similarly, Jewish people believe that Muslims believe in the same God that they do and Muslims also believe that they believe in the same God as Judaism. However, Christians do not believe that they believe in the same God as Muslims and many Muslims do not believe that Christians believe in the same God as they do.
To me, I believe that if you believe in one all mighty God then you believe in the same God as I do - but your worship is different. Do Muslims and Christians believe in the same God?