While Judaism and Christianity share a great deal of beliefs, the main difference between the two religions is squarely focused on what each religion believes about Jesus Christ.
Both Christianity and Judaism believe in the biblical foundations and stories written about in the Old Testament. There are some differences in exactly which books get included and their importance to the overall religion, and these differences result in differences in practices, such as which holidays (holy days) get celebrated.
It is the existence and importance of Jesus Christ that is main separator of the two religions. Both Christians and Jews believe that Jesus existed; however, Jews believe that Jesus was a completely mortal man and born of two human parents. Christians believe that Jesus was immaculately conceived by the Holy Spirit and born of the virgin Mary. Christians also believe that Jesus is both completely human and completely God and that this allowed him to live a completely sinless and holy life.
Both religions recognize that Jesus died, but Christians believe that Jesus was resurrected three days after being crucified. His death and resurrection were necessary in order to save people from their sins. For Christians, it is through God's grace and faith in Jesus Christ that they can obtain eternal salvation. Judaism does not recognize the divinity of Jesus Christ, and salvation involves a much more legalistic following of rules.