Panel Moderation is an important function. If you have a large panel, and/or a large audience, also having someone who is skilled at moderating can be more critical to the success of your panel than putting yet another person on the panel who is an expert on the topic. A good moderator keeps the conversation flowing and can even rescue a panel that starts off with half the panelists saying "I don't know why I'm here" (though the head of programming should try to avoid those in the first place).
The key to panel moderation is there in its name: to ensure that each panelist produce a moderate proportion of the panel content while also trying to facilitate a panel that is successful on the whole. Doing this may require drawing out quiet or reserved panelists, curtailing the loquaciousness of the more talkative panelists, asking questions to motivate the discussion or keep it on track, and enforcing a policy that is appropriate to the panel regarding questions from the audience.
See also Not a Question, More of a Comment: Killing the "Naqmoac", an essay on panel moderation by John Scalzi.
For the Beginning Moderator
If you are a moderator, a good way to start off is by announcing what the title of the panel is, reading the Panel Blurb out of the Program Book, and then having each panelist introduce themselves. Some moderators also ask the panelists to answer an initial question as part of their introduction; answers to these questions help the moderator know which way the panelists are going to jump on an issue, and can help get the ball of conversation rolling.
Note that if you are assigned to moderate a panel and would like to get more traction beforehand about who the panelists are and what they think would be fun to talk about, two terrific ways include discussing the topic beforehand by email, and meeting shortly before the panel in the Green Room to talk about it. Googling your fellow panelists can also be a good idea, given sufficient lead time and interest.
Consider finding out or deciding at the beginning whether the panel wants to take questions throughout, or at the end; then let the audience know what the plan is. As the moderator, it is your job to track how often people have asked questions and (roughly) who has been waiting how long to speak. It is fairly common and well-supported to accept a question from someone who has not said or asked anything yet over someone who has already had a turn. If you have a good memory for it, it can be nice to indicate the next few people in the queue (e.g. "Next will be you, you in the blue, and you, and I have noticed you in the back, don't worry, I'll get to you") so they can take their hands down. If a particular topic inspires a new rush of hands, one can also ask for questions on that topic before going to questions that shift the discussion to a new topic.