The square kilometre is a unit of area and is a multiple of the square metre; an SI derived unit. It has the symbol km².
A square kilometre is equal to the area of a square with edges 1km long. There are 1,000,000 square metres in a square kilometre.
Square kilometres are often used to describe expansive areas of land due to the large unit size.
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.