Judaism: Monotheistic, ritualistic, and the base of Christianity and Islam. Christianity: Monotheistic (while subtly tri-theistic), very open, and spawned from Judaism. Islam: Ditto, but with an Arabic flair. These are essentially the basic similarities of Judaism, Christianity, and Islam. Judaism is the original of these three while the latter two were created from the Judaism. While they are very much the same, they are very much different. One of the things I've learned from planning this essay is that I really don't know much about Islam. While I've been brought up being taught Christianity (which includes Judaism as, essentially, a prelude) and fully understand the principles and intricacies, I've never actually read the Koran (even though we have a copy) or deeply studied Islam. That said, here are the contrasts of Judaism and Christianity.
First and most obvious is the fact that all three are monotheistic. In other words, they all worship and believe there is only one deity (officially anyways). In Judaism, God is the creator and gave Moses the code of conduct, or, the Torah to follow to achieve life after death. Christianity and Islam believe the same but they both believe that God abandoned the "law" (Torah) because of the stubbornness of the Jews. In Christianity, God sent his only son Jesus Christ as a human being to teach the coming the kingdom and present a new covenant and a new path to heaven. Islam differs in that the main messenger of Islam was an Arab prophet named Mohammed who taught that Muslims should pledge allegiance to Allah (the Arabic word for "God"). They too believe that God chose Abraham to breed the Hebrews but they believe that his true first son, Ishmael, was destined to become Allah's chosen people. In Islam you get to heaven by following the Five Pillars of Islam.
Besides being monotheistic, they also believe that God is the only god. In all three they believe that God is the "alpha and omega, the beginning and end" and that he alone created the universe and everything in it.