how is it classified?
Stratus, cirrus, stratocumulus, cumulus, and cumulonimbus.
Cloud
It became the basis of a modern international system that divides clouds into five physical "forms" which can be further divided or classified into altitude "levels" to derive ten basic "genera". The main representative cloud types for each of these forms are stratus, cirrus, stratocumulus, cumulus, and cumulonimbus. 
chemical and physical processes include the formation of clouds On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture (usually in the form of water vapor) from an adjacent source to raise the dew point to the ambient temperature.
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Cloud [SEP] Introduction
"Low-level" clouds do not have any altitude-related prefixes. However "mid-level" stratiform and stratocumuliform types are given the prefix "alto-" while "high-level" variants of these same two forms carry the prefix "cirro-". Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as "multi-level" or "vertical". Most of the ten genera derived by this method of classification can be subdivided into "species" and further subdivided into "varieties". Very low stratiform clouds that extend down to the Earth's surface are given the common names "fog" and "mist", but have no Latin names.