Originally the term neo-conservative referred to people who were previously leftists or socialists who had come into the conservative movement. Usually they were formerly intrigued by communism, but having seen it close up, had become strongly anti-communist. They may or may not have adopted every bit of the conservative view, but they were outspokenly anti-communist and so, since that was the life-and-death-struggle we were in at the time, they were the conservatives best friend.
The better known "neo's" were Jewish.
The term has recently become a pejorative as some folks who consider themselves conservative, but are anti-Israel, or for other reasons oppose the wars in Afghanistan and Iraq, have begun to try and make the case that only they are genuine conservatives. Pat Buchanan is one, for example. Since, the left has chimed in, and has started using the same term in reference to any conservative who either supports the war in Iraq or Afghanistan, or supports Israel. The first implication is that such people are not really conservative. The other, underlying implication, is that they are Jewish.
Some folks like to make the case that Bush and the US government are controlled by the Jews, without actually coming out and saying it. Their choice of the word "neo-conservative" allows them to make a fairly explosive charge without having to defend it; if you call them on it, they can always retreat and claim that they didn't mean what you say they mean.
Others will try to use the term to say that "neo's" pretend to be conservative but are really secret socialists or communists. The proof of that? The proof is that they support the war, or they support Israel. So, you see where it goes. In its original meaning, it had meaning. As presently used by Buchanan, the left, and journalists in general, it just means you are a member of a secret cabal that supports Bush.