Condensing

When a gas is exposed to cold air, the energy in the particles start to slow down, the particles come together, and soon form a liquid

Water vapor is a good example to use when explaining condensation. Water vapor is a gas because it has no definite shape, no definite volume, it can be compressed and it can flow quite easily.
When water vapor is introduced to a cold air, it's particles will come together and form a liquid (which in this case would be water).
To turn a gas back into a liquid we use condensation.

Water vapor condensing on a plastic water bottle