The meaning of the word, sophist has changed greatly over time. Initially, a sophist was someone who gave sophia to his disciples, i.e., wisdom made from knowledge. It was a highly complimentary term applied to early philorophers such as the Seven Wise Men of Greece.
In the second half of the 5th entury B.C., and especially at Athens, "sophist" came to be applied to a group of thinkers who employed debate and rhetorc to teach and disseminate their ideas and offered to teach these skills to others. Due to the importance of such skills in the litigious social life of Athens, practitioners of such skills often commanded very high fees. The practice of taking fees, coupled with willingness of many practitioners to use their rhetorical skills to pursue unjust lawsuits, eventually led to a decline in respect for practitioners of this form of teaching and the ideas and writings associated with it.
Protagoras is generally regarded as the first sophist. Other leading 5th-century sophists included Gorgias and Prodicus. Socrates was perhaps the first philosopher to significantly challenge the Sophists...
The Sophists held a relativistic view on cognition and knowledge. Their philosophy contains criticism of religion, law and ethics. Though many sophists were as religious as their contemporaries, some helf atheistic of agnostic views.
Unfortunately most of the original texts written by the sophists have been lost, and modern understanding of sophistic movement comes from analysis of Plato's writings. It is necessary to keep in mind that Plato and the sophists had severe ideological differences, and Plato might have benefited from modifying or slanting the original sophistic arguments when he presented them in his writings (ironically, a sophistic technique at work), or may even not have fully understood their arguments himself. An excellent book on the topic is "The Sophistic Movement" by G.B. Kerferd.
In the Roman Empire, sophists were just professor of rhetoric. For instance, Libanius, Himerius, Aelius Aristides and Fronto were considered sophists in this sense.
(retrieved from Wikipedia --the Free Encyclopedia http://en.wikipedia.org/wiki/Sophist)