Afternoon tea is definitely the lighter of the two, taken at 3 or 4 by
the middle classes, whereas high tea is the main evening meal for the
working classes. It's also known as just "tea" in the north of England.

There's a lot of dispute about the origin of the term "high" but I've
always gathered it refers to the "high" or late part of the afternoon
when it's taken (around 5 or 6).

--Paul