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 hectare is a unit of area and is a non-SI but metric unit with the symbol ha.
It represents the area equal to that of a square with edges 100m long (ie 1hm² or 10,000m²). This is the equivalent to about 2.47 acres.
Its size is down to its use primary use; that for measuring the size of land.