The are is a unit of area and is a non-SI but metric unit with the symbol a.
It measures exactly 100m² which is the equivalent to 0.0247 acres.
The are is the principle unit for measuring the size of land across the world.
The square metre is a unit of area and is an SI derived unit with the symbol m².
It was introduced in 1954 when the International System of Units was established and was derived directly from one of the original 6 base units - the metre (m).
It can be prefixed with any of the SI symbols that multiply it up or down such as km² (square kilometre) or µm2 (square micrometre). It is the most commonly used unit of area and is recognised all over the world.