A gas needs a container, often with a lid, to keep it in. Most gases will escape through any opening in a container.
The air that we breathe is made up of lots of different ones, such as oxygen, carbon dioxide, and nitrogen. Some gases are very light, such as helium, and we use it to fill special party balloons, because it makes the balloon float.
Gases are invisible and often don't have a smell.
Some gases are explosive when a match is brought near, like methane, that we use in our cookers and central heating systems, so a horrible smell is added so that people know when it leaks.