By definition, High Tea and Afternoon Tea are 2 entirely different things. Though, in short, in Hong Kong they are used interchangeably. Read on if you're interested.
Contrary to popular belief (well, popular belief in Hong Kong anyway), "high" tea isn't named so because of the tall tiered stand, nor because it was made for people on the higher end of the social scale (it wasn't - see below). So strictly speaking, teas at hotels served on a stand should be known as "afternoon tea" rather than "high tea".
History has it that the idea of afternoon tea comes from the upper classes in England, and typically refers to the meal taken mid-afternoon. It could include things like pastries and sandwiches and be served on a tiered stand, as many hotels like to do, although this is not obligatory.
High tea, on the other hand, is a meal originating from the 'lower' classes, who needed to fill up after a hard day's work. High tea occurs a bit later, around 5pm, and would (traditionally) usually replace dinner.
Of course, all this is open to discussion. This is just what I've read from various sources over the years. You can probably find out more about it on Wikipedia or something.
Anyway, in Hong Kong, you can assume that High Tea and Afternoon Tea mean pretty much the same thing. Although local cafés (the Cha Chaan Teng, or Chinese cafés, that Sheeana mentioned) are unlikely to call their afternoon discount meals "high tea".