The Rhetorical Triangle
LOGOS:  How can I make the argument internally consistent and logical? How can I find the best reasons and support them with the best evidence?
(aka "Arguments Based on Facts and Reasons)
PATHOS:  How can I make the reader open to my message? How can I best appeal to my reader's values and interests? How can I engage my reader emotionally and imaginatively?
(aka "Arguments from the Heart")

Writer or Speaker
ETHOS:  How can I present myself effectively? How can I enhance my credibility and trustworthiness?
(Arguments Based on Character)
Effective arguments should consider all three points on this rhetorical triangle.   
Ethos:  refers to the credibility of the writer/speaker and is often conveyed through the tone and style of the message, through the care with which the writer considers alternative views, and through the investement in his or her claim.  In some cases, it is also a function of the writer's reputation for honesty and expertise independent of the message. 
Pathos:  Pathos is often associated with emotional appeal and appeals more specifically to an audience's imaginative sympathies--their capacity to feel and see what the writer feels and sees.  Thus when we turn the abstractions of logical discourse in a palpable and immediate story, we are making a pathetic appeal.
Logos:  Refers primarily to the internal consistency and clarity of the message and to the logic of its reasons and support.