A gas is anything that has no definite volume and takes the shape of it's container. Gas molecules move all around their container very fast because they are not connected.
This diagram shows the arrangment and movement of gas molecules
The nitrous oxide in a canister of whipped cream is an example of a gas
The carbon monoxide that comes out of a tailpipe is a second example of a gas
The helium in a hot air balloon is another example of a gas