No difference...
48ci is 0.8l
A 48 cubic inch tank (commonly referred to as a steely) will hold 0.8 litres of compressed air at 3000psi.
Or you may be looking at a fibre wrapped tank, an aluminium core wrapped in fibre glass and usually covered in protective plastic. These, known as carbon fibre tanks are a lot lighter than steel bottles and can also hold 1.5 times the air... e.g a 48ci fibre tank will hold 0.8 l of air up to 4500 psi.