Theism, in the broadest sense, is the belief that at least one deity exists. In a more specific sense, theism is a monotheistic doctrine concerning the nature of a god or goddess, and their relationship to the universe. Theism, in this specific sense, conceives of God as personal, present and active in the governance and organization of the world and the universe. As such theism describes the classical conception of God that is found in Christianity, Judaism, Islam, Sikhism and some forms of Hinduism. The use of the word theism to indicate this classical form of monotheism began during the scientific revolution of the seventeenth century in order to distinguish it from the then-emerging deism which contended that God, though transcendent and supreme, did not intervene in the natural world and could be known rationally but not via revelation.
The term theism derives from the Greek theos meaning "god". The term theism was first used by Ralph Cudworth (1617–88). In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things". Atheism is rejection of theism in the broadest sense of theism; i.e. the rejection of belief that there is even one deity. Rejection of the narrower sense of theism can take forms such as deism, pantheism, and polytheism. The claim that the existence of any deity is unknown or unknowable is agnosticism. The positive assertion of knowledge, either of the existence of gods or the absence of gods, can also be attributed to some theists and some atheists. Put simply theism and atheism deal with belief, and agnosticism deals with (absence of) rational claims to asserting knowledge.
Credit with notations: http://en.wikipedia.org/wiki/Theism