There is a common core to the concept of God as it is understood by Judaism, Christianity, and Islam. According to this conception, God is:

the creator of the universe
omnipotent (all-powerful)
omniscient (all-knowing)
perfectly just
personal --
    possessing consciousness
    possessing beliefs and desires
    possessing love and special concern for human beings
    an agent who interacts with human beings
omnipresent*

*Although the Judeo-Christo-Islamic tradition does not regard God as immanently omnipresent (actually located everywhere), it does regard God as transcendently omnipresent (located beyond space and time but nonetheless capable of seeing and appearing everywhere).