A cloud is a visible mass of condensation droplets or ice crystals suspended in the atmosphere above the surface of the Earth or another planetary body.
On Earth, the condensing substance is water vapor, which forms small droplets of water (typically 0.01 mm or ice crystals that, when surrounded with billions of other droplets or crystals, are visible as clouds. Clouds reflect all visible wavelengths of light equally and are usually white, but they can appear grey or even black if they are so thick or dense that sunlight cannot pass through.
Cloud formation and properties
 
A variety of cloud formations.
 
Global scheme of cloud optical thickness.
Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. This can happen in two ways.
1. The air is cooled to its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen
along warm and cold fronts (Frontal lift),
where air flows up the side of a mountain and cools as it rises higher into the atmosphere (orographic lift),
by the convection caused by the warming of a surface by insolation (diurnal heating),
when warm air blows over a colder surface such as a cool body of water.
2. The air stays the same temperature but absorbs more water vapour into it until it reaches saturation.
Clouds are quite heavy. The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of water vapor is actually low enough that air currents below and within the cloud are capable of keeping small droplets suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This give these droplets plenty of time to re-evaporate as they fall into in the warmer air beneath the cloud.
Most water droplets are formed when water vapor condenses around a condensation nucleus, a tiny particle of smoke, dust, ash, or salt. In supersaturated conditions, water droplets may act as condensation nuclei.
Water droplets large enough to fall to the ground are produced in two ways. The most important is through the Bergeron Process, theorized by Tor Bergeron, in which supercooled water droplets and ice crystals in a cloud interact to produce the rapid growth of ice crystals, which precipitate from the cloud and melt as they fall. This process typically takes place in clouds with tops cooler than -15°C. The second most important process is the collision and wake capture process, occurring in clouds with warmer tops, in which the collision of rising and falling water droplets produces larger and larger droplets, which are eventually heavy enough to overcome air currents in the cloud and the updraft beneath it and fall as rain. As a droplet falls through the smaller droplets which surround it, it produces a "wake" which draws some of the smaller droplets into collisions, thus perpetuating the process. This method of raindrop production is the primary mechanism in low stratiform clouds and small cumulus clouds in tropical regions. It typically produces smaller raindrops and drizzle.
The actual form of cloud created depends on the strength of the uplift and on air stability. In unstable conditions convection dominates, creating vertically developed clouds. Stable air produces horizontally homogeneous clouds. Frontal uplift creates various cloud forms depending on the composition of the front (ana-type or kata-type warm or cold front). Orographic uplift also creates variable cloud forms depending on air stability, although cap cloud and wave clouds are specific to orographic clouds.

Cloud Classification
 
Cloud Classification by altitude of occurrenceClouds are divided into two general categories: layered and convective. These are named stratus clouds (or stratiform, the Latin stratus means layer) and cumulus clouds (or cumiloform, cumulus means piled up). These two cloud types are divided into four more groups that distinguish the cloud's altitude. Clouds are classified by the cloud base height, not the cloud top. This system was proposed by Luke Howard in 1802 in a presentation to the Askesian Society.

High clouds (Family A)
These generally form above 16,500 feet (5,000 m), in the cold region of the troposphere. However, in Polar regions they may form as low as 10,000 ft (3,048 m). They are denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent.