Recently, Howard Pepper contacted me in response to the articles that I've been writing about John the Baptist for Bible Study and the Christian Life (see The Popularity of John the Baptist; The Proclamation of John the Baptist; and Was the Prophesy of John the Baptist Fulfilled?). He wanted to know if I'd be interested in dialoging about the Baptist via our blogs (we might call it "diablogging"). I thought that this could be an enjoyable exercise since I'm already doing a lot of studying about this important figure and it is nice to talk to people with shared interest. We decided to begin with Josephus' passing description of the Baptist since this is a lesser known perspective when compared with that presented in the early Christian Gospels. Eventually we will make our way through the Gospels and the Book of Acts and then maybe even further examining later literature.
This is the above mentioned text from Josephus in Antiquities 18.5.109-119 that mentions the Baptist (if you don't want to read the whole excerpt, the second paragraph describes John and I summarized the first paragraph below):
(109) About this time Aretas (the king of Arabia Petrea) and Herod had a quarrel, on the account following: Herod the tetrarch had married the daughter of Aretas, and had lived with her a great while; but when he was once at Rome, he lodged with Herod, who was his brother indeed, but not by the same mother; for his Herod was the son of the high priest Simon’s daughter. (110) However, he fell in love with Herodias, this last Herod’s wife, who was the daughter of Aristobulus their brother, and the sister of Agrippa the Great. This man ventured to talk to her about a marriage between them; which address when she admitted, an agreement was made for her to change her habitation, and come to him as soon as he should return from Rome; one article of this marriage also was this, that he should divorce Aretas’s daughter. (111) So Antipas, when he had made this agreement, sailed to Rome; but when he had done there the business he went about, and was returned again, his wife having discovered the agreement he had made with Herodias, and having learned it before he had notice of her knowledge of the whole design, she desired him to send her to Macherus, which is a place on the borders of the dominions of Aretas and Herod, without informing him of any of her intentions. (112) Accordingly Herod sent her thither, as thinking his wife had not perceived anything; now she had sent a good while before to Macherus, which was subject to her father, and so all things necessary for her journey were made ready for her by the general of Aretas’s army and by that means she soon came into Arabia, under the conduct of the several generals, who carried her from one to another successively; and she soon came to her father, and told him of Herod’s intentions. (113) So Aretas made this the first occasion of his enmity between him and Herod, who had also some quarrel with him about their limits at the country of Gamalitis. So they raised armies on both sides, and prepared for war, and sent their generals to fight instead of themselves; (114) and, when they had joined battle, all Herod’s army was destroyed by the treachery of some fugitives, who, though they were of the tetrarchy of Philip, joined with Aretas’s army. (115) So Herod wrote about these affairs to Tiberius; who, being very angry at the attempt made by Aretas, wrote to Vitellius, to make war upon him, and either to take him alive, and bring him to him in bonds, or to kill him, and send him his head. This was the charge that Tiberius gave to the president of Syria.
(116) Now, some of the Jews thought that the destruction of Herod’s army came from God, and that very justly, as a punishment of what he did against John, that was called the Baptist; (117) for Herod slew him, who was a good man, and commanded the Jews to exercise virtue, both as to righteousness towards one another, and piety towards God, and so to come to baptism; for that the washing [with water] would be acceptable to him, if they made use of it, not in order to the putting away [or the remission] of some sins [only], but for the purification of the body; supposing still that the soul was thoroughly purified beforehand by righteousness. (118) Now, when [many] others came in crowds about him, for they were greatly moved [or pleased] by hearing his words, Herod, who feared lest the great influence John had over the people might put it into his power and inclination to raise a rebellion (for they seemed ready to do anything he should advise), thought it best, by putting him to death, to prevent any mischief he might cause, and not bring himself into difficulties, by sparing a man who might make him repent of it when it should be too late. (119) Accordingly he was sent a prisoner, out of Herod’s suspicious temper, to Macherus, the castle I before mentioned, and was there put to death. Now the Jews had an opinion that the destruction of this army was sent as a punishment upon Herod, and a mark of God’s displeasure against him." 
In summary, Herod Antipas the Tetrarch over Galilee and Perea is married to the daughter of King Aretas IV, the ruler of the Nabateans (see the map for regions). While visiting his half-brother Herod Philip I in Rome he falls in love with his wife, Herodias, who was also the daughter of another half-brother, Aristobulus. Herodias agrees to marry Antipas if he divorces his wife. The news of this arrangement found its way back to Antipas' wife who requested of Antipas that she might spend some time at Macherus, a fortress near the border shared with the Nabateans.  Since Antipas did not know that she knew of his plans he had no idea that she wanted to go to Macherus in order to slip back into her father's territory. Once she had done this it was possible for Aretas IV to war against Antipas, and he did, and he defeated Antipas' forces soundly.
Where does the Baptist come into the picture? Well, Josephus says that Antipas executed John because of John's sway over the people. He believed that they would listen to anything he said, which could destabilize the region, especially if John called for some sort of revolt against Antipas. So, Antipas executed John. When Antipas was defeated by the Nabateans the people remembered this event, which may have been several years earlier, and they said that it was God's vengeance on John's behalf that caused the outcome.
There remains a lot more to be said, most importantly about how the Baptist is described, but I want to give Howard Pepper a chance to respond with any insights or questions he might have about this section. His blog is Natural Spirituality. When he has posted his introduction (i.e., Pt. 1b) I will link to it here then respond at a later date.
 Flavius Josephus and William Whiston, The Works of Josephus: Complete and Unabridged (Peabody: Hendrickson, 1987). Italics mine.
 For a more detailed description see Peter Richardson, Herod: King of the Jews and Friend of the Romans (Columbia: University of South Carolina Press, 1996), 306-307.