Afternoon Tea is a light tea served about 4:00 pm to hold you over to a later dinner. It consists of delicate sandwiches, scones, jam and clotted cream, and small cakes. Obviously served with a pot of tea.
High Tea as I remember it as a child was usually what we ate on a Saturday or Sunday late afternoon. We always had a roast Sunday dinner, so late Sunday afternoon was a High Tea for us. Consisting of possibly a meat pie or a fish pie and salad, kippers with bread and butter, cold meats chese and salad items, served with salad cream. Followed by scones and cakes and a pot of tea.
There were of course lots of variations on high tea, but it's a more substantial meal.
The link above gives some nice details and history.