Athe·ist : one who does not believe in the existence of any gods. (pronunciation: 'A-thE-ist)

The·ist : one who believes in the existence of a god or gods; specifically : one who believes in the existence of a personal God viewed as the creator of man and the world. (pronunciation: 'thE-ist)

Ag·nos·tic : one who believes that it is impossible to know whether there is a God or gods, that it is unknown and probably unknowable; yet does not profess true atheist. (pronunciation: ag-'näs-tik)

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Eth·i·cal Athe·ist :  one who does not believe in the existence of any gods and feels a personal duty and obligation to live by a set of principles, standards of conduct and philosophy in hopes of living in peace with fellow human beings.  (pronunciation: 'e-thi-k&l 'A-thE-ist)

Note: The "Ethical Atheist" definition is generally compatible with the "Atheist" definition; and even redundant in most cases.  Yet, just as there are unethical theists, there are unethical atheists.  So, "Ethical Atheist" refers to what we believe to be the large majority of atheists.  For more see "What's an Ethical Atheist?"