The gas inside fluorescent light bulbs is a mixture of argon and mercury vapor. The argon is to make sure there is no oxygen or regular atmosphere in the bulbs, while the mercury is what provides the actual light.
Yes, there can be other gases in very small amounts. They are used to change the color of light emitted.
What other gases?
They're called phosphors and will usually be the iodide of rare earth metals like yttrium and europium. These two make the light more blue or red when added.
How does the gas work?
When we send an electric current through mercury vapor, it becomes excited. This causes it to put out photons, the units of light itself. When these photons react with the phospors, they become the light in the color that we can see from the bulb.
If we had just air and mercury in the fluorescent bulb then we would have to use much more power to get the light, if it worked at all. The argon is therefore used to make the entire system more efficient.
But isn't mercury poisonous?
Yes, but very little is used in each fluorescent light bulb and it does not leak. If you break one it is best to open the windows and leave the room for a few minutes. Also, be careful not to touch the broken glass with your bare hands. But you're not going to do that anyway, because it is broken glass, right?