Aluminum cutters are typically used in milling machines or machining centres to perform milling operations. They remove material by their movement within the machine or right from the shape of the precision cutting tools.
Ball end mills are end mills that refer to rounded cutters or radiused cutters. They are usually made from high speed steel or carbide. They are the most common tool used in a vertical mill. When professionals need a ball end mill they will know to look for a tool with a rounded cutter instead of the average ones that are flat bottomed.
While you are putting your ball end mills in Cat 40 tool holders you can considering getting a slot drill. Slot drills are center cutting endmills, generally two, but it could be three or four, fluted cutters that are capable of drilling straight down into the material and then moving laterally to cut a slot.
Ball end mills could actually mean two different things but the main meaning is a that are also used to add a radius between perpendicular faces to reduce stress concentrations. Ball end mills is just a slang way of saying ball nose cutter. HSK tool holders can hold your different types of cutters. Helpful info also found here.