[Molasar]

simplest way if looking at is this:
TEA is a drink that can only be made by pouring boiling water onto tea leaves (sorry America, a cup of hot water with a tea bag on the side IS NOT tea, no way Jose - and it doesn't matter where or how much you paid, that's indisputably not tea).
AFTERNOON TEA is a pot of TEA with a light plate of cold sandwiches, cake, scones and similar items.
HIGH TEA is based around TEA and is AFTERNOON TEA plus light cooked hot meal items to support you until dinner, I guess that was necessary in the days 150 years ago before central heating, when people moved around or on horseback instead of sitting on their arses or in cars like we do now.
DINNER was the main meal of the day consisting of soup, fish, roast meats, trimmings and as many vegetable accompaniments as you could manage, dessert, cheese. Plus wines to suit each course.
SUPPER was a light snack meal usually cold for when one returned home from one's evening out, say to DINNER or the theatre or the tavern. Or of course one could miss supper and have a nightcap from the grog* tray.

yeah, I know, those old buggers really knew how to live.

*grog in this case does not refer to the Royal Navy rum ration issued to ratings.