Wikileaks is a website that was set up by Julian Assange (an australian) to provide people with an anonymous avenue to leak government information into the public domain. It has operated for a long time largely without the scrutiny of the US government.
The reason Assange and his site have found their way into the news recently is because of Pvt. Bradley Manning, an American soldier initiated the dissemination of hundreds of thousands of classified documents, and a number of secret documents to the public domain.
The government is using a classic concept, postulated in the film, Wag the dog, to distract the american public. They have successfully guided the media into efforts to demonise Assange, and have brushed the Pvt. Manning issue under the rug.
They want people to say, "Damn that Assange! He is an international terrorist bent on our destruction!"
rather than having people ask, "How did an entry level member of the US army manage to access State department cables, and information that is considered top secret, over the defense department network"
As for questions 3 and 4, The lion's share of the information has been trivialities, like reports about Qaddafi and his host of blonde Ukrainian nurses, but the major concern that many members of government have, relates to the information that has yet to be released. Assange has promised that if he is prosecuted within the United States (Currently it doesn't seem like he is eligible to be prosecuted under the espionage act) his supporters will release thousands of documents, considerably more damaging than the first leaks.
The truth of the damage that has been done, is that it is not as extensive as it at first seems. Countries like Iran and North Korea, who hold the US in low esteem, are simply finding that the feeling is mutual. Countries that are allied with the US in its global endeavours, are hardly going to cease the cooperation because some state department official said that members of their government were party animals.