It is often confusing to Americans when they hear the worda High or Afternoon Tea. The common misconception is that High Tea is a formal event attended by the upper crust and that Afternoon Tea is a leisure time in the middle of the day where tea and sweets are served. The truth is that Afternoon Tea is the more formal of the two.
Afternoon Tea became popular among the upper classes as a sort of stop gap between lunch which is served around noon or 1 pm, and supper which is served around 7 or 8. With up to an 7 hour gap betwen meals, you can imagine that one might get hungry between them. The meal that is now known as Afternoon Tea was introduced for that purpose.
Afternoon Tea is a light meal, traditionally consisting of an assortment of sweets and/or light sandwiches, designed to tide one over until supper, not to fill the stomach. As an institution of the upper classes it naturally developed a very formal and ceremonious nature.
High Tea, on the contrary, is a working class meal designed to replace dinner or supper. Served between 6 and 7 in the evening the designation of "High" is believed to refer to the high backed chairs of the lower classes or the high stools found in pubs where the meal was sometimes served and not to the ceremony or formality. Someimes the meal is also referred to as "Meat Tea" designating that it is a heavier meal which included more sustenance (such as meat) for working class men and women as opposed to the frilly light fare of Afternoon Tea.