They are called slots.

I'm going to assume you have a set of drill bits, a drill press, and files. Find a drill bit that has the diameter of the width of your intended slot. Drill holes at the very ends of where you want your slot to be. Depending on how long your slot is you might drill some additional holes between the end holes. Then file away until it looks like a slot. The cheap way to do it is to try to cut sideways with your drill bit but its not recomended. Drill press bearings are not meant to take a side load and neither are jobber drill bits. It might be ok with really thin aluminum or plastic but its not good practice.

With a mill you would simply clamp your workpice on a set of parallels, the intended slot being parallel to the X or Y axis. Then you would plunge down with your slot or end mill, move the table to your desired slot length, and be finished.