Hi caps are High Capicity magazines that usually hold 200+ bb's that are winded up in the bottom of the mag. BB's are easy to fill up because of its trap door that you can easily feed bb's in. They usually have feed problems like double feeding and winding them is a bit annoying.
Low caps are Low Capacity magazines that hold 30-80 bb's that are spring loaded, which means you have to load them in with a loading rod or a mag loader. They do not double feed and feed without the use of winding gears. Usually for the Mil-simmer.