what is a gas?
A gas has no definite shape and no definite volume. This is because the space between the molecules in a gas is very large and this allows the gas to extend because of conditions around it. A gas doesn't keep its shape just like a liquid.
molecules in a gas
http://www.chem.purdue.edu/gchelp/atoms/states.html
Molecules in a gas are spread apart. They move randomly and rapidly past each other. It's hard to think of a gas and imagine particles because you can't really see a gas. But imagine, rapid moving particles when you at least think of a gas.
examples of a gas
As pictured above, you can see a hot air ballon is a gas. But not the whole thing, just the hot air. Compressed air is also a gas, so that means fire extinguishers are filled with a gas. So is smoke, that's a gas you can see.