Along with openness, conscientiousness, extraversion, and neuroticism, agreeableness is one of the big five factors of personality. Agreeableness refers to qualities such as kindness, dependability, and cooperation. Personality traits like agreeableness describe core characteristics about the makeup of a person's personality.
Conscientiousness is the personality trait of being careful, or vigilant. Conscientiousness implies a desire to do a task well, and to take obligations to others seriously. Conscientious people tend to be efficient and organized as opposed to easy-going and disorderly.
Dr. Dombeck's Note: To clarify just a bit, neuroticism is the first and most major personality factor that falls out of the 5 factor model. It differentiates people on the basis of whether, by nature, they are calm or alarmist. Neuroticism is *highly* correlated with anxiety and depression.