Clouds, whether they are big gray layers, fluffy white lumps, or streaks in the sky from aeroplanes, all have a few things in common. Most importantly, they are made up of tiny droplets of water, called "cloud droplets." Cloud droplets form when hot air from the Earth's surface rises and slowly cools down. Like the particles, the cloud droplets are so small that they do not fall to the ground (unless the cloud is raining). The rising air around them is pushing them up, gravity is pulling them down, and they end up just floating around the sky.
In the sky, unless there is some kind of surface for it to stick to, or unless it is really, really cold (-38°C), water exists as a gas. When there is a surface, water can liquefy (become liquid) or freeze onto the surface, as it does on the ground. However, in the sky, there are no big obvious surfaces. Therefore, to make a cloud droplet, you need a tiny particle, which has a small surface that water can stick to. The particle acts as the seed for the droplet.

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