A gas is one of the different phases of matter. Similar to liquids and plasmas, gases are fluids. This means that they have the ability to flow and that they will not usually return to their prior stage after they have been deformed. This doesn't mean that they don't have viscosity. A property of a gas that is different from that of a liquid is that; a gas does not occupy a fixed space or volume, but will instead expand to fill whatever container it is in, this is opposite of a liquid that will remain the same.
After Plasma, gas has the second highest energy per molecule. Due to this extreme amount of kinetic energy, gas atoms and molecules of gas will usually bounce off any container they are placed in, and even off of eachother. This bouncing will occur with more power as the kinetic energy of the gas is increased. Many people, incorrectly believe that when these molecules collide provide the main basis in explaining gas pressure. This is not true because the random velocities of the gas are enough to define the gas pressure. Not to leave these collisions in the dark, as they are very important in establishing the Maxwell-Boltzmann distribution of the gas.
It's interesting how we use the word "gas" every single day, and all that we relate it to is the rising gas prices, or something that we get when we eat too many beans. There is alot more to gas then that, and is an important part of our everyday lives.