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.
The square centimetre is a unit of area and is a multiple of the square metre; an SI derived unit. It has the symbol cm².
A square centimetre is equal to the area of a square with edges 1cm long. There are 10,000 square centimetres in a square metre.
It is often used to describe areas smaller than 1m² - including much work in classrooms on paper.