The acre is a unit of area and is used in the imperial and US customary systems with the symbol a.
It represents the area the size of one chain by one furlong (66 by 660 feet). This is exactly equal to 10 square chains, 1⁄640 of a square mile or 43,560 square feet.
It is generally used to measure the size of land.
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.