Welcome to Gaia! ::

Heard about them.
What exactly are they? o:

Not only are panels like Glacirus said...
But they are also informative about culture.

There are many Lolita Panels and Cosplay Panels that show you the how-to's and such of fashion and costuming.

Our local con is even planning on having a Shogi and a Go Panel to teach con-goers how to play.

A con in our state is actually starting Activity Panels.
These panels require a small fee to attend but you come out with something cool in hand.

For example: I'm excited about the sushi panel.
You pay a few bucks and they show you how to make your own sushi. I already know how to roll and handle sushi but you can't pass up the chance to make your own sushi with others! >w<

They are also having a wig styling panel where you pay a small fee and get a training wig ((cheap wig)) to learn first hand how to style a wig.

How would cosplayers get their own panels?
[i.e. Parle Productions]

Adding onto what Charmful IncarnatE said, signing up to do a panel isn't always guaranteed. I know that Otakon does this, but I do not know what other anime conventions do.

For Otakon, you apply to do a panel. The staff reviews all panel applications and the staff chooses which panels will make an appearance at the anime convention that year.