Previous posts in this series were 1) Looking for a Messiah and 2) The Restoration of Israel.
Into this context comes John the Baptist. Of all the Jewish groups of the day, he looks the most like an Essene. He is off in the desert wearing the clothes of a prophet like Elijah. He is actually baptizing not far from Qumran, a conservative Essene community.  He is preaching the approaching kingdom of God, the restoration of God's people. Once again, of all the Jewish groups we know from the time, the Essenes most seem to have had this emphasis from time to time.
But many Pharisees probably did too. Of course, most Jews in Palestine did not belong to any group of this sort. At their peak, there may not have been more than 6,000 Pharisees and 4,000 Essenes. Like today, most people were simply doing their best to get by, farming their land and feeding their children. We should not think that the latest Pharisaic idea on purity made the average dinner conversation of your average Jew at the time, any more than philosophy is a big dinner conversation in most of our homes.
John arguably baptized at about the place where Joshua led Israel into Canaan. In itself this speaks of the restoration of the nation. The gospels remember him by way of texts like Isaiah 40:3-5. Although New Testament authors did not always think of the original context of a passage when they quoted them, the context of Isaiah 40 fits John's situation well. Isaiah 40 was originally a call to captive Israel in Babylon to come home from exile. Be comforted, Israel, for God is calling you home. Make a bee line through the desert. Lower the mountains, raise the valleys, straighten out the road so that you can get home as quickly as possible.
Matthew, Mark, and Luke also remember John in relation to Malachi 3:1, whose context is a messenger who comes to warn that God is coming to judge his people and the temple in particular. The Essenes in particular had serious problems with the high priests and temple administration of their day. In fact, their very origins may very well have involved a split with the temple, with their founding leader coming out on the losing side of the argument. 
These two Scriptures alone give us a very plausible sense of what John the Baptist was doing there along the Jordan River. He was a messenger announcing both the coming judgment of God on Israel for its sin yet also proclaiming its coming restoration as a nation. The appropriate response was repentance for sin, accompanied by a symbolic washing in the river that symbolized Israel's transition from the wilderness to the land. The baptism stood for washing their sins, with God's forgiveness accompanying.
The Jewish historian Josephus mentions John the Baptist.  As we would expect, he sterilizes his description somewhat, arguably softening some of the more nationalistic dimensions to John's message. So John preached virtue, righteousness toward one another, and piety toward God. Josephus removes the concrete element of Israel's restoration as a nation. Even here, however, he mentions that John's baptism had a politically threatening element. Josephus gives the potential for rebellion as the reason Herod Antipas put him to death.
John did not invent this sort of ritual washing. You will find miqvaot all over Israel at the time. These were baths for a person to purify themselves from all the different sorts of impurities found in Leviticus, like uncleanness from touching a dead body. These were large enough to walk down into and be immersed, and probably "baptism" usually did involve immersion at the time.
However, a couple aspects of John's baptism seem quite unusual. For one thing, John's baptism was not about ceremonial uncleanness. It was a cleansing for sin, which would normally involve sacrifices in the temple. Of course if John did not accept the current temple as legitimate, we can see how baptism for him and possibly at Qumran might arise as a temporary but appropriate substitute for sacrifice in the temple.
A second difference from the normal ritual washings is that John's baptism had somewhat of the nature of a one time event. It is not that John necessarily would have refused a person who returned a second time but that the baptism was in preparation for the Day of the Lord, not an act to be repeated every time you sinned. It was thus an "apocalyptic" baptism, one that related to soon coming, world changing events.
 There were many more Essenes than lived at Qumran on the northwest side of the Dead Sea. The best scholarship on the subject now consider those who lived there to be just one group of Essenes--probably a more conservative and sectarian group than other Essenes who lived in cities. Some Essenes married; some did not. Think of how there are more and less conservative denominations within the same Christian traditions, such as the difference between United Methodist and Wesleyan or between Wesleyan and Bible Methodist.
 For an excellent overview of the Dead Sea Scrolls and the Essenes, see James VanderKam's The Dead Sea Scrolls Today.
 Josephus, Antiquities 18.5.2.